Fix a crash when checking a type of an unbound variable
Fix a crash when checking a type of an unbound variable

Previously executing `:t x` in an enivornment where `x` hasn't been
defined would crash the REPL because `!` errors out when the key is not
in the map. Replacing `!` with a safer lookup function fixes the issue.

google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang