feat(tazjin/rlox): Implement expression statements

These aren't particularly useful without side effects, but one step at
a time.

This diverges slightly from the book, in that OpPop retains the last
value it "forgot" from the stack in a special field on the
interpreter.

This makes it possible to return values from expression statements,
which helps in cases where Lox is embedded as a scripting
language (please don't do this ever) or in tests.

Change-Id: Ided0bc04c6e80ddb23ba4693d61ac9e08b002d58
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2584
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

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

@@ -28,4 +28,5 @@ pub enum OpCode {
 
     // Built in operations
     OpPrint,
+    OpPop,
 }