Glean-parser

- Add support for generating C\ code.
- BUGFIX: The memory unit is now correctly passed to the MemoryDistribution
metric type in Swift.

- BUGFIX: look for metrics in send\_if\_empty pings. Metrics for these kinds of
pings were being ignored.

- Support for Python 3.5 has been dropped.
- BUGFIX: The ordering of event extra keys will now match with their enum,
fixing a serious bug where keys of extras may not match the correct values in
the data payload. See <https://bugzilla.mozilla.org/show_bug.cgi?id=1648768>.

- **Breaking change:** (Swift only) Combine all metrics and pings into a single
generated file `Metrics.swift`.

- `glinter` messages have been improved with more details and to be more
actionable.
- A maximum of 10 `extra_keys` is now enforced for `event` metric types.
- BUGFIX: the `Lifetime` enum values now match the values of the implementation
in mozilla/glean.

- BUGFIX: yamllint errors are now reported using the correct file name.