Open-code collection data structure rather than using Dictionary+List

This rewrites the core of the type to be based on a custom data structure rather than wrapping Dictionary and List. The core structure is based on both Dictionary in corelib and the OrderedDictionary prototype in corefxlab.

This also adds missing TryAdd, Capacity, EnsureCapacity, and TrimExcess members for parity with Dictionary, and fixes debugger views for the Key/ValueCollections.

Author: stephentoub

src/libraries/System.Collections/ref/System.Collections.cs

@@ -116,6 +116,7 @@ public OrderedDictionary(System.Collections.Generic.IEnumerable<System.Collectio
         public OrderedDictionary(System.Collections.Generic.IEqualityComparer<TKey>? comparer) { }
         public OrderedDictionary(int capacity) { }
         public OrderedDictionary(int capacity, System.Collections.Generic.IEqualityComparer<TKey>? comparer) { }
+        public int Capacity { get { throw null; } }
         public System.Collections.Generic.IEqualityComparer<TKey> Comparer { get { throw null; } }
         public int Count { get { throw null; } }
         public TValue this[TKey key] { get { throw null; } set { } }
@@ -142,6 +143,7 @@ public void Add(TKey key, TValue value) { }
         public void Clear() { }
         public bool ContainsKey(TKey key) { throw null; }
         public bool ContainsValue(TValue value) { throw null; }
+        public int EnsureCapacity(int capacity) { throw null; }
         public System.Collections.Generic.KeyValuePair<TKey, TValue> GetAt(int index) { throw null; }
         public System.Collections.Generic.OrderedDictionary<TKey, TValue>.Enumerator GetEnumerator() { throw null; }
         public int IndexOf(TKey key) { throw null; }
@@ -170,6 +172,8 @@ void System.Collections.IDictionary.Remove(object key) { }
         void System.Collections.IList.Insert(int index, object? value) { }
         void System.Collections.IList.Remove(object? value) { }
         public void TrimExcess() { }
+        public void TrimExcess(int capacity) { }
+        public bool TryAdd(TKey key, TValue value) { throw null; }
         public bool TryGetValue(TKey key, [System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out TValue value) { throw null; }
         public partial struct Enumerator : System.Collections.Generic.IEnumerator<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.IDictionaryEnumerator, System.Collections.IEnumerator, System.IDisposable
         {

src/libraries/System.Collections/src/System.Collections.csproj

@@ -9,13 +9,11 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <Compile Include="System\Collections\ThrowHelper.cs" />
     <Compile Include="System\Collections\BitArray.cs" />
-    <Compile Include="$(CoreLibSharedDir)System\Collections\Generic\DebugViewDictionaryItem.cs"
-             Link="Common\System\Collections\Generic\DebugViewDictionaryItem.cs" />
-    <Compile Include="$(CoreLibSharedDir)System\Collections\Generic\ICollectionDebugView.cs"
-             Link="Common\System\Collections\Generic\ICollectionDebugView.cs" />
-    <Compile Include="$(CoreLibSharedDir)System\Collections\Generic\IDictionaryDebugView.cs"
-             Link="Common\System\Collections\Generic\IDictionaryDebugView.cs" />
+    <Compile Include="$(CoreLibSharedDir)System\Collections\Generic\DebugViewDictionaryItem.cs" Link="Common\System\Collections\Generic\DebugViewDictionaryItem.cs" />
+    <Compile Include="$(CoreLibSharedDir)System\Collections\Generic\ICollectionDebugView.cs" Link="Common\System\Collections\Generic\ICollectionDebugView.cs" />
+    <Compile Include="$(CoreLibSharedDir)System\Collections\Generic\IDictionaryDebugView.cs" Link="Common\System\Collections\Generic\IDictionaryDebugView.cs" />
     <Compile Include="System\Collections\Generic\LinkedList.cs" />
     <Compile Include="System\Collections\Generic\OrderedDictionary.cs" />
     <Compile Include="System\Collections\Generic\PriorityQueue.cs" />
@@ -29,14 +27,10 @@
     <Compile Include="System\Collections\Generic\StackDebugView.cs" />
     <Compile Include="System\Collections\StructuralComparisons.cs" />
     <!-- Shared Common -->
-    <Compile Include="$(CoreLibSharedDir)System\Collections\HashHelpers.cs"
-             Link="Common\System\Collections\HashHelpers.cs" />
-    <Compile Include="$(CommonPath)System\Collections\Generic\BitHelper.cs"
-             Link="Common\System\Collections\Generic\BitHelper.cs" />
-    <Compile Include="$(CommonPath)System\Collections\Generic\EnumerableHelpers.cs"
-             Link="Common\System\Collections\Generic\EnumerableHelpers.cs" />
-    <Compile Include="$(CommonPath)System\Obsoletions.cs"
-             Link="Common\System\Obsoletions.cs" />
+    <Compile Include="$(CoreLibSharedDir)System\Collections\HashHelpers.cs" Link="Common\System\Collections\HashHelpers.cs" />
+    <Compile Include="$(CommonPath)System\Collections\Generic\BitHelper.cs" Link="Common\System\Collections\Generic\BitHelper.cs" />
+    <Compile Include="$(CommonPath)System\Collections\Generic\EnumerableHelpers.cs" Link="Common\System\Collections\Generic\EnumerableHelpers.cs" />
+    <Compile Include="$(CommonPath)System\Obsoletions.cs" Link="Common\System\Obsoletions.cs" />
   </ItemGroup>
 
   <ItemGroup>