Implement hash function

Duplicates improvement made to Base's Enums: JuliaLang/julia#30500

**Before:**
```julia
julia> @bitflag Flag1 flag1=0 flag2 flag3 flag4

julia> @benchmark hash(flag1)
BenchmarkTools.Trial: 10000 samples with 1000 evaluations.
 Range (min … max):  11.035 ns … 52.801 ns  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     15.714 ns              ┊ GC (median):    0.00%
 Time  (mean ± σ):   16.003 ns ±  1.305 ns  ┊ GC (mean ± σ):  0.00% ± 0.00%

                                        ▁▃ █▁▄        ▁
  ▂▂▂▁▁▁▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▄▂▃▄██▆███▃▃▂▂▂▂▆▅█▆▆█▅▃▂ ▃
  11 ns           Histogram: frequency by time        17.5 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.
```

**After:**
```julia
julia> @benchmark hash(flag1)
BenchmarkTools.Trial: 10000 samples with 1000 evaluations.
 Range (min … max):  1.397 ns … 33.455 ns  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     4.819 ns              ┊ GC (median):    0.00%
 Time  (mean ± σ):   4.907 ns ±  1.042 ns  ┊ GC (mean ± σ):  0.00% ± 0.00%

                                               ██    ▅▅
  ▂▁▁▁▁▂▂▂▁▁▁▁▂▃▂▁▁▁▁▂▂▁▁▁▂▃▃▂▁▁▁▂▂▂▁▁▁▁▂▂▂▁▁▃███▆▁▂▄███▃▁▂▂ ▃
  1.4 ns         Histogram: frequency by time        5.66 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.
```

Author: jmert

src/BitFlags.jl

@@ -239,6 +239,9 @@ macro bitflag(T::Union{Symbol,Expr}, syms...)
         BitFlags.haszero(::Type{$(esc(typename))}) = $(esc(maskzero))
         Base.typemin(x::Type{$(esc(typename))}) = $(esc(typename))($lo)
         Base.typemax(x::Type{$(esc(typename))}) = $(esc(typename))($hi)
+        let flag_hash = hash($(esc(typename)))
+            Base.hash(x::$(esc(typename)), h::UInt) = hash(flag_hash, hash(Integer(x), h))
+        end
         let insts = (Any[$(esc(typename))(v) for v in $(values)]...,)
             Base.instances(::Type{$(esc(typename))}) = insts
         end

test/runtests.jl

@@ -62,6 +62,10 @@ end
     @test Flag1(7) & flag1a == flag1a
     @test flag1a < flag1b < flag1c
     @test flag1a | flag1b < flag1c
+
+    # Hashing
+    @test Int(flag2a) == Int(flag3a)    # same numerical value, but
+    @test hash(flag2a) != hash(flag3a)  # unique hashes as BitFlag
 #end
 
 #@testset "Type properties" begin