Move desugaring to new roc_can_solo crate

[smores56]

We eventually want to split all canonicalization that can be done solo into the roc_can_solo crate and put whatever else remains into a roc_can_combine crate. As a preliminary step, this change pulls just the initial desugaring work out from canonicalization into roc_can_solo. There is some cruft (e.g. unnecessary cloning of data, some dead code, etc.) added here, but it should make for an easier transition to the new state of canonicalization. Some notes on the changes:

• The desugaring changes look massive, but really, I just moved everything to the new crate except for the desugar_invalid_dbg_expr code since it's used in the middle of canonicalization and not at the beginning like the other code.
• I didn't set up really for caching besides the bare minimum; we have intentionally pulled out caching into a separate issue since this change, though focused around enabling caching, will work without it, and that issue should be a good learning opportunity for a new compiler contributor.