Move slice functions to vtable, implement hash

include/grpc/impl/codegen/slice.h

include/grpc/slice.h

src/core/ext/transport/chttp2/transport/hpack_encoder.c

src/core/lib/iomgr/resource_quota.c

src/core/lib/slice/slice.c

src/core/lib/slice/slice_hash_table.c

src/core/lib/slice/slice_intern.c

src/core/lib/transport/static_metadata.c

tools/codegen/core/gen_static_metadata.py