Allow negative key ids

[rooftopsparrow]

Hello thriftrw!

We have older thrift files that used to allow negative key. The Apache thrift compiler has a command line argument to allow theses to compile:

--allow-neg-keys  Allow negative field keys (Used to preserve protocol
                compatibility with older .thrift files)

I was curious if this was possible to support this behavior in this project as well ( optionally via a constructor flag allowNegativeKeys: true perhaps )?

It appears as though the "no negative keys" are enforced by an assert AND in the bufrw project as well at first glance, but I have not done a deep investigation.

If this is something you'd like to support I'd love to contribute to the effort. Thanks!

[kriskowal]

We would accept an allowNegativeKeys contribution. Be sure to thread it through new Thrift for submodules.