feat(tvix/eval): implement simple arithmetic binary operations

Implements simple arithmetic operations (+, -, *, /).

There is some scaffolding included to pop and coerce pairs of numbers,
as the Nix language will let arithmetic operators apply to arbitrary
pairs of number types (always resulting in floats if the types are
mixed).

Change-Id: I5f62c363bdea8baa6ef812cc64c5406759d257cf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6074
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/eval.rs

@@ -12,7 +12,7 @@ pub fn interpret(code: String) -> EvalResult<String> {
     }
 
     let mut out = String::new();
-    writeln!(out, "{}", ast.root().dump()).ok();
+    println!("{}", ast.root().dump());
 
     let code = crate::compiler::compile(ast)?;
     writeln!(out, "code: {:?}", code).ok();