More typechecking error message improvements

[byorgey]

Towards #1297 . In this PR:

• Moves some functions out of Pipeline and into more appropriate modules
• Adds a new type Join to represent two types or other things, one "expected" and one "actual", at the point where they join + we check whether they are equal; a bunch of refactoring to use this new type. This way we don't have to remember which argument is which when calling a function that takes one "expected" thing and one "actual" thing.
• Refactors all the decomposeXXX functions to take a Syntax node to use in error messages, and to take a Sourced UType (i.e. a UType along with whether it is "expected" or "actual") instead of just a UType so we can generate accurate error messages
• Introduce a basic typechecking stack that keeps track of "what we are currently in the middle of checking", so that when we encounter an error we can say things like "while checking the definition of foo". Currently this is just a basic proof of concept.
• A few other miscellaneous error message improvements.

[kostmo]

FYI rebasing on main will fix the restyled error.