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/value.rs

@@ -1,9 +1,29 @@
+use super::interner::InternedStr;
+
 #[derive(Clone, Debug, PartialEq)]
 pub enum Value {
     Nil,
     Bool(bool),
     Number(f64),
-    String(String),
+    String(LoxString),
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum LoxString {
+    Heap(String),
+    Interned(InternedStr),
+}
+
+impl From<String> for LoxString {
+    fn from(s: String) -> Self {
+        LoxString::Heap(s)
+    }
+}
+
+impl From<InternedStr> for LoxString {
+    fn from(s: InternedStr) -> Self {
+        LoxString::Interned(s)
+    }
 }
 
 impl Value {