*: update to grpc 1.26.0

[BusyJay]

Close #375.

[hicqu]

grpc_slice grpc_slice_new_with_len(void* p, size_t len,
                                   void (*destroy)(void*, size_t)) {
  grpc_slice slice;
  slice.refcount = (new grpc_core::NewWithLenSliceRefcount(destroy, p, len))
                       ->base_refcount();
  slice.data.refcounted.bytes = static_cast<uint8_t*>(p);
  slice.data.refcounted.length = len;
  return slice;
}

Seems we can pass capacity into grpc_slice_new_with_len, and then

slice.data.refcounted.length = v.len();

[BusyJay]

I prefer not to rely on the implement details of grpc_slice_new_with_len.

[hicqu]

I think just not to rely grpc_core::NewWithLenSliceRefcount is ok. slice.data.refcounted.length is public for us, so we can adjust it if only we can ensure it's less than capacity.
Otherwise memory copy is necessary, I think we need to avoid it.

[BusyJay]

If not rely on grpc_core::NewWithLenSliceRefCount, then you can't for sure whether slice.data.refcounted.length or its internal field is forwarded to destroy hook.

[nrc]

lgtm

[BusyJay]

@nrc @hicqu PTAL.

One breakage is that grpcio probably can't link statically on MacOS that is <= 10.13. A similar issue can be found alexcrichton/curl-rust#279.

[nrc]

One breakage is that grpcio probably can't link statically on MacOS that is <= 10.13

Seems OK, I think most Mac devs stay up to date.