Rewrite core transformer

[phischu]

This should fix #1158

[b-studios]

I tried to reduce as many eta-redexes as possible without breaking backwards compatiblity. Some eta-redexes are purposefully not covered in this PR:

• eta redexes around resume (which might require a redesign)
• eta redexes around pattern matching cases (where the right hand sides are lifted to local definitions)

All others should have been eliminated now. If you still find some, please let me know.

[b-studios]

@phischu once CI passes this is ready to merge