refactor: use named rustemo actions, add compiler context

[LeanSerra]

Description

Modify grammar.rustemo file to use named rules, this way instead of having confusing names for the possible rules:

pub enum Expressions {
    Expression(Expression),
    C2(ExpressionsC2),

we have more useful names like

pub enum Expressions {
   ExpressionSingle(Expression),
   ExpressionRecursive(ExpressionRecursive),
}

Also moves all the compiler context to a file named context.rs that will initialize all the necessary context for the compiler inside a thread_local! macro
Finally this PR adds the recursive rule for declaring multiple variables in a single line like a, b, c : int this is required for creating the symbol table.

[MatiBarcia]

LGTM