Support implicit field keys

[jparise]

Fields without explicit keys are automatically assigned starting from -1
and working their way down. Implicit field keys are deprecated by Apache
Thrift, but supporting them gives us greater Thrift IDL compatibility.

[jparise]

@maralla and @hit9, do you think this is something you'd consider for review and merging? It's an old Thrift feature, but we have some .thrift files that rely on it, and we unfortunately can't just add the explicit negative key values to the IDL files without triggering other warnings.

[jparise]

@maralla and @hit9, just wanted to follow-up and ask if this is still under consideration.

[jparise]

Sorry to bother @lxyu @hit9 @maralla, but would you consider merging this?