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

Cargo.toml

@@ -4,4 +4,4 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
-rust_decimal = { version = "1.43.0", features = ["macros"] }
+rust_decimal = { version = "1.43.0", features = ["macros", "maths"] }