feat(eval): add extremely basic evaluation

added a basic `eval_expr` function that evaluates an expression.
it still does not reduce/simplify anything, and it cannot evaluate to fractions yet

src/math/mod.rs

@@ -1,6 +1,7 @@
 use crate::math::types::Expression;
 
 mod display;
+mod eval;
 mod lexer;
 mod parser;
 
@@ -11,3 +12,7 @@ pub fn parse(source: &str) -> Expression {
     let mut parser = parser::from(lexer.collect());
     parser.parse()
 }
+
+pub fn eval(expr: Expression) -> Expression {
+    eval::eval_expr(expr)
+}