Bugfix wrong message name in a verifier error
EDocument forgotten cmd line option -descr
Insert an empty line after generated functions

This is to make it so that different functions do not bump into each
other, by separating them a little
Refactor codegen transform: keep erl_syntax tree format

Retain the erl_syntax parse-tree format for as long as possible,
revert to the erl_parse parse-tree format only as the last
step. Previously, this was done early and often, now it is either
deferred to the latest possible moment (for gpb_codegen:mk_fn)
or not needed at all (for gpb_codegen:format_fn).

The intention of keeping the erl_syntax parse-tree format, is to be
able to make use of the erl_syntax:text() feature, in order to make an
attempt at generating code for maps instead of records.
Add support for maps

Generates code that works with maps as defined in EEP 43 in
www.erlang.org/eeps/eep-0043.html, as of eep repo version bf2d098.