Add Source Code Typing Tool

[Vici37]

This PR introduces a new crystal tool apply-types subcommand as described in this crystal forum post and initially implemented in this other repo as a stand alone tool. A tool that fully types a crystal file is obviously very tightly coupled with the language and compiler semantic, and so having this tool be part of the compiler itself feels like the correct direction.

That being said, I'm not 100% it's ready yet, but I do think it's ready enough at least to solicit feedback from anyone who is willing to provide it :)

[crysbot]

This pull request has been mentioned on Crystal Forum. There might be relevant details there:

https://forum.crystal-lang.org/t/exploring-the-compiler/7343/21

[nobodywasishere]

This is awesome!