Modules reorganisation.

Moved everything from interpreter/* to the repository root. Placed all
sources in src/ and all tests in test/ directories. Hierarchy in test/
reflects the one in src/.

Moved modules related to lucretia language into src/Lucretia/*. Moved
Definitions, Syntax and Types into src/Lucretia/Language/*. Moved
Parser, Interpreter and TypeChecker into src/Lucretia/. Moved utility
modules into src/Util/*.