Simplify folder tree

Get rid of unnecessary “environment/” folder level in repository (a
remnant of the original project).
Add lexeme, lexer, and types modules
Add parser and trees modules
Add evaluator and environment modules
Add function-evaluator and loader modules
Add built-ins and built-in-types modules
Add runner module and makefile
Create README.md

Only most basic information; will merge with existing readme.txt after filling repoistory.