feat(tazjin/rlox): Intern all string constants

This is again a step closer to the book, but there are some notable
differences:

* Only constants encountered by the compiler are interned, all other
  string operations (well, concatenation) happen with heap objects.

* OpReturn will always ensure that a returned string value is newly
  heap allocated and does not reference the interner.

Change-Id: If4f04309446e01b8ff2db51094e9710d465dbc50
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2582
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/bytecode/mod.rs

@@ -27,7 +27,7 @@ impl crate::Lox for Interpreter {
         &mut self,
         code: String,
     ) -> Result<Self::Value, Vec<Self::Error>> {
-        let chunk = compiler::compile(&code)?;
-        vm::interpret(chunk).map_err(|e| vec![e])
+        let (strings, chunk) = compiler::compile(&code)?;
+        vm::interpret(strings, chunk).map_err(|e| vec![e])
     }
 }