Introducing new lexer/parser for Eolian

This set of changes introduces a new, handwritten lexer+parser in Eolian. This should
allow for easier maintenance as well as easier feature additions. Unlike the previous
effort, this separates lexer and parser part (but they still run at the same stage).

In long term some more APIs should get exposed (for IDEs and other tools). The parser
is also stricter and more conformant to the actual format so several issues with
existing Eo files were caught previously.

Merges branch 'devs/q66/eolian_new_parser'

@feature