Initial elimination of grpc_mdstr from metadata.{h,c}

include/grpc/slice.h

src/core/lib/slice/slice_traits.h

src/core/lib/transport/metadata.c

src/core/lib/transport/metadata.h

src/core/lib/transport/static_metadata.c

src/core/lib/transport/static_metadata.h

tools/codegen/core/gen_static_metadata.py