Wrapped compile() body in el_arena_push/pop so the arena is active
before lex() and scan_fn_sigs(). Previously both ran with
_tl_arena_active=0, leaking all token and signature strings permanently.
Also prevents inner pop(mark=0) calls from deactivating the arena
between per-function scopes. Verified: self-host PASS, RSS stable.
Will Anderson
e8f6765750