feat(tazjin/rlox): Add initial support for strings

... including concatenation.

This diverges significantly from the book, as I'm using std::String
instead of implementing the book's whole heap object management
system.

It's possible that Lox in Rust actually doesn't need a GC and the
ownership model works just fine.

Change-Id: I374a0461d627cfafc26b2b54bfefac8b7c574d00
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2577
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

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

@@ -3,6 +3,7 @@ pub enum Value {
     Nil,
     Bool(bool),
     Number(f64),
+    String(String),
 }
 
 impl Value {