Handle fragmented messages from F* by storing fragments in a buffer

[klinvill]

Resolves #18.

Messages from F* larger than 8192 bytes seem to get fragmented into multiple messages when read by the language server. This typically results in invalid JSON messages when this fragmentation occurs. This PR supports re-assembly of fragmented messages under the assumptions that:

• fragmented messages will always be delivered sequentially and in-order
• messages that are not valid JSON are always fragmented messages

I'm opening this as a draft PR since I'd like to add unit tests for my changes, but am running into some difficulties that might be best addressed by refactoring (e.g. loading the server module that contains the handlers currently requires a connection for the server).

[nikswamy]

Thanks Kirby! This looks great.

If there's a way to add a unit-testing framework that would be amazing and useful beyond the scope of this PR. Unfortunately, at the moment, my testing is limited to trying out the extension for a bit and seeing if it works for a bit ... clearly inadequate.

I'll try your PR out to see if it works well for me. If it does, I'd be open to merging it, while perhaps separately we try to figure out what a unit test framework for this extension would look like.

[klinvill]

Ok sounds good! I've been working on some potential refactoring that I think should make it easier to unit test (and mock out the I/O bits), but it's turning into quite the overhaul. At this point that probably belongs in a separate PR.

[nikswamy]

Thanks Kirby. Finally got around to testing this. It works well for me ... merging.