Baseline: a0cd3553

Important changes:

  - json.decode now takes an optional `default` positional parameter;
    if this parameter is specified and decoding fails, json.decode
    will return
    the `default` value instead of failing Starlark evaluation.

This release contains contributions from many people at Google, as well as Benjamin Peterson, David Ostrovsky, eareilg, Fabian Meumertzheim, Son Luong Ngoc.