Add errors to the lexer
Parse ID0
Some modifications of the lexer.
[work in progress] Writting th parser.
Parser almost complete.
Improve error reporting.
Fix a parser bug.
Fix another parser bug.
Improved error handling in parser (use $startpos / $endpos rather than Parsing which is deprecated).
Solved a shift/reduce conflict.
Added strings to lexer
Merge branch 'LexPars' into MLF

Conflicts:
	Makefile
	ast.ml
	lexer.mll
	parser.mly
	utils.ml
Add typing phase. Add support for (mutually) recursive definitions in the typer.