Using Bigstringaf as the internal buffer

[wokalski]

Hey, I'm using h2 with gRPC and I'm giving ocaml-protoc a try now. In order to limit copies and allocations I'm wondering if we could have a variant of pbrt that uses Bigstringaf for the internal buffer.

I mean I know we can, and it seems like a low hanging fruit, but maybe there's a reason it's a dumb idea that I'm not seeing.