Evaluate memory region expressions using definitions

[diorcety]

Based on PR #7, adding the support of defined variable inside memory origin / length

[pftbest]

Thanks @diorcety ! Although the PARSE_STATE in a thread local storage looks a bit strange to me. Have you considered just running the evaluation as a second pass over the Vec<RootItem> after the old parser finished? Or there is some special case where you need to know the results during parsing, and cannot delay it till the second pass?

[pftbest]

For example, extend the Region struct like this:

#[derive(Debug, PartialEq)]
 pub struct Region {
     pub name: String,
     pub origin: u64,
     pub length: u64,
+    pub origin_expr: Expression,
+    pub length_expr: Expression,
 }

And only write the expressions during parsing. Then on a second pass calculate origin and length based on that expressions.