Several ADT improvements

[Felalolf]

This PR adds several requested improvements for adts and fixes some past issues with adts:

• the result of an adt constructor is now the declared type of the adt type definition (was a special clause type before)
• arguments to adt literals now consider implicit conversions
• match expressions can occur in assertions
• the names of adt clause fields can be omitted
• duplicate field names now cause a type error instead of a crash
• duplicate clause names now cause a type error instead of a crash
• adt clause name can be qualfied over the adt type
• adt clause names do not cause name collision with other package entities (their precedence is set to lowest)
• set and sequence operations (except conversions) are now more forgiving
• importing adts is a bit more robust
• fixed an issue with reorderings of match statement clauses

Adt clauses imported from another package can now be unreachable. Currently, qualified names of types must have at most one dot. However, with the new design, accessing an adt clause may require 2 dots, e.g. pkg.adtType.adtClause. This should be fixed in a future PR.

Fixes #686
Fixes #685
Fixes #641
Fixes #589

[jcp19]

Patina! I just have a few minor comments