implements parsing

the lexer sends tokens to the parser, (lex_this and lex_next variables)
which will add some expressions or evaluate from the t_stack.

For example :

the expression is 1 + 2 * 3 + 6

add 1
add +
add 2
add *
add 3

evaluate the 2 last numbers

(so now the stack is 1 + 6)

add 6

evaluate the 2 last numbers

(so now the stack is 1 + 12)

evaluate the 2 last numbers

the results is 13.

note that no operations are implemented yet.