Preserve &[T]'s Rust representation in rust::Slice

[dtolnay]

#645, but for slices too.

Part of #608.

[dtolnay]

MSVC doesn't like something:

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