Introduce BytesBuilder

[antocuni]

#127 and #179 made it official that we can't create "half-ready" strings which are supposed to be filled after creation.

However, there are cases in which this is a legitimate use case and currently it is not supported by HPy. We should:

1. introduce StringBuilder, similar to the existing TupleBuilder and ListBuilder
2. modify the error message raised by HPyBytes_FromStringAndSize to point the user to the right solution. Something like ... please use StringBuilder instead.

[cklein]

I started writing some different implementations for a PoC (https://github.com/cklein/hpy/tree/cklein-stringbuilder), but maybe the people who started the project or have been contributing for a longer time have a clearer vision of how the API should look like.

What would be the basic operations?
Would you expect the operands to be HPyUnicode objects or c strings?

[TeamSpen210]

Would it be worth it to add a 2-stage accumulator, like _PyAccu? That's used for StringIO and the json module, it periodically takes the small strings accumulated and compacts them into single large strings. That way building a large string it doesn't have too much overhead.

[hodgestar]

@TeamSpen210 That sounds like a cool optimization to have under the hood, although I wouldn't make it part of the API (not that you or CPython seemed to be suggesting that).

[antocuni]

note: the actual API to use for this is being discussed in #214

[Albert55554]

Need to discuss cpython