Merge 'Expression equality checking, some optimizations' from Preston Thorpe

This PR mainly adds custom logic to check equality in ast expressions.
Not sure if this belongs in the `vendored` parser or not, let me know
and I'll bring it out. Also replaces `Vec` arguments with slice refs
where possible, as well as some clippy warnings in the same `emitter`
file.
I'll write some more tests tomorrow to make sure this is as thorough as
possible.
EDIT: failed test same issue referenced in #484. Marking as draft until
more tests + cases added

Reviewed-by: Jussi Saurio <jussi.saurio@gmail.com>

Closes #488

testing/shelltests.py

@@ -3,7 +3,7 @@ import os
 import subprocess
 
 # Configuration
-sqlite_exec = "./target/debug/limbo"
+sqlite_exec = os.getenv("SQLITE_EXEC", "./target/debug/limbo")
 cwd = os.getcwd()
 
 # Initial setup commands