Literals and variable binding parsing

[cvauclair]

The Mesh Engine should be able to parse any expressions consisting of basic types literals and variable bindings to them.

Basic types literals:	Type	Literal
unit	()
int	whole number (e.g.: 21, -124978)
float	decimal number (e.g.: 0.9123, -123.752)
bool	true or false
string	sequance of ascii characters delimited by double quotes (e.g.: "asdf")

Variable bindings:

let u = ();       // unit
let i = 2;        // int
let f = 2.0;      // float
let b = true;     // bool
let s = "Hello";  // string