Notable changes:
* `prost` crates are now edition 2018
* Code generation is now compatible with edition-2018 crates, as well as
edition-2015 crates. No explicit configuration is necessary to enable
edition-2018 support.
* Reserved field names now use raw identifiers instead of adding an
underscore.
* The tests crate now executes for both 2018 and 2015 edition via
separate crates which reference the same source files.