Fix slice compilation on MSVC

    cxxbridge-demo.lib(blobstore.o) : error LNK2019: unresolved external symbol "void __cdecl sliceInit(void *,void const *,unsigned __int64)" (?sliceInit@@YAXPEAXPEBX_K@Z) referenced in function "public: __cdecl rust::cxxbridge1::Slice<unsigned char const >::Slice<unsigned char const >(unsigned char const *,unsigned __int64)" (??0?$Slice@$$CBE@cxxbridge1@rust@@qeaa@PEBE_K@Z)
    cxxbridge-demo.lib(blobstore.o) : error LNK2001: unresolved external symbol "void __cdecl sliceInit(void *,void const *,unsigned __int64)" (?sliceInit@@YAXPEAXPEBX_K@Z)
    cxxbridge-demo.lib(main.rs.o) : error LNK2001: unresolved external symbol "void __cdecl sliceInit(void *,void const *,unsigned __int64)" (?sliceInit@@YAXPEAXPEBX_K@Z)
    cxxbridge-demo.lib(main.rs.o) : error LNK2001: unresolved external symbol "void __cdecl sliceInit(void *,void const *,unsigned __int64)" (?sliceInit@@YAXPEAXPEBX_K@Z)
      Hint on symbols that are defined and could potentially match:
        "void __cdecl rust::cxxbridge1::sliceInit(void *,void const *,unsigned __int64)" (?sliceInit@cxxbridge1@rust@@YAXPEAXPEBX_K@Z)
    cxxbridge-demo.lib(blobstore.o) : error LNK2019: unresolved external symbol "unsigned __int64 __cdecl sliceLen(void const *)" (?sliceLen@@YA_KPEBX@Z) referenced in function "public: unsigned __int64 __cdecl rust::cxxbridge1::Slice<unsigned char const >::size(void)const " (?size@?$Slice@$$CBE@cxxbridge1@rust@@QEBA_KXZ)
    cxxbridge-demo.lib(main.rs.o) : error LNK2001: unresolved external symbol "unsigned __int64 __cdecl sliceLen(void const *)" (?sliceLen@@YA_KPEBX@Z)
      Hint on symbols that are defined and could potentially match:
        "unsigned __int64 __cdecl rust::cxxbridge1::sliceLen(void const *)" (?sliceLen@cxxbridge1@rust@@YA_KPEBX@Z)
    cxxbridge-demo.lib(blobstore.o) : error LNK2019: unresolved external symbol "void * __cdecl slicePtr(void const *)" (?slicePtr@@YAPEAXPEBX@Z) referenced in function "private: void * __cdecl rust::cxxbridge1::Slice<unsigned char const >::ptr(void)const " (?ptr@?$Slice@$$CBE@cxxbridge1@rust@@AEBAPEAXXZ)
    cxxbridge-demo.lib(main.rs.o) : error LNK2001: unresolved external symbol "void * __cdecl slicePtr(void const *)" (?slicePtr@@YAPEAXPEBX@Z)
      Hint on symbols that are defined and could potentially match:
        "void * __cdecl rust::cxxbridge1::slicePtr(void const *)" (?slicePtr@cxxbridge1@rust@@YAPEAXPEBX@Z)
    D:\a\cxx\cxx\target\debug\deps\demo.exe : fatal error LNK1120: 3 unresolved externals

Author: dtolnay

include/cxx.h

@@ -180,8 +180,6 @@ class Slice final
   void swap(Slice &) noexcept;
 
 private:
-  void *ptr() const noexcept;
-
   std::array<std::uintptr_t, 2> repr;
 };
 
@@ -487,26 +485,27 @@ constexpr unsafe_bitcopy_t unsafe_bitcopy{};
 
 #ifndef CXXBRIDGE1_RUST_SLICE
 #define CXXBRIDGE1_RUST_SLICE
+void sliceInit(void *, const void *, std::size_t) noexcept;
+void *slicePtr(const void *) noexcept;
+std::size_t sliceLen(const void *) noexcept;
+
 template <typename T>
 Slice<T>::Slice() noexcept {
-  void sliceInit(void *, const void *, std::size_t) noexcept;
   sliceInit(this, reinterpret_cast<void *>(align_of<T>()), 0);
 }
 
 template <typename T>
 Slice<T>::Slice(T *s, std::size_t count) noexcept {
-  void sliceInit(void *, const void *, std::size_t) noexcept;
   sliceInit(this, const_cast<typename std::remove_const<T>::type *>(s), count);
 }
 
 template <typename T>
 T *Slice<T>::data() const noexcept {
-  return reinterpret_cast<T *>(this->ptr());
+  return reinterpret_cast<T *>(slicePtr(this));
 }
 
 template <typename T>
 std::size_t Slice<T>::size() const noexcept {
-  std::size_t sliceLen(const void *) noexcept;
   return sliceLen(this);
 }
 
@@ -523,7 +522,7 @@ bool Slice<T>::empty() const noexcept {
 template <typename T>
 T &Slice<T>::operator[](std::size_t n) const noexcept {
   assert(n < this->size());
-  auto pos = static_cast<char *>(this->ptr()) + size_of<T>() * n;
+  auto pos = static_cast<char *>(slicePtr(this)) + size_of<T>() * n;
   return *reinterpret_cast<T *>(pos);
 }
 
@@ -663,7 +662,7 @@ bool Slice<T>::iterator::operator>=(const iterator &other) const noexcept {
 template <typename T>
 typename Slice<T>::iterator Slice<T>::begin() const noexcept {
   iterator it;
-  it.pos = this->ptr();
+  it.pos = slicePtr(this);
   it.stride = size_of<T>();
   return it;
 }
@@ -679,12 +678,6 @@ template <typename T>
 void Slice<T>::swap(Slice &rhs) noexcept {
   std::swap(*this, rhs);
 }
-
-template <typename T>
-void *Slice<T>::ptr() const noexcept {
-  void *slicePtr(const void *) noexcept;
-  return slicePtr(this);
-}
 #endif // CXXBRIDGE1_RUST_SLICE
 
 #ifndef CXXBRIDGE1_RUST_BOX