Lint functions with exactly the same body

[clarfonthey]

This could probably be an AST check; it doesn't have to be complicated. Just a really simple thing to check if something is an obvious copy-paste error. If two functions have the same body, the user should just call one from the other, or change the bodies so they're not the same.

[oli-obk]

Should be two lints. One that checks types, and one that doesn't. The one that doesn't should be allow by default.

[killercup]

Dupe of #719?

…also bonus points for integrating with incremental compilation which collects a lot of metadata and hashes and is probably well on its way to do that even on a function level

[oli-obk]

Kind of a dupe. Let's keep two different issues. One for MIR one for AST