- fix a bug in Positive.decompose that was introducing components that were not
in the original type
- rewrite Positive.decompose to flatten long chains of `Cup [`Cup[`Cup ...]]
- rewrite apply_raw to perform the correct dove-tail exploration of the branches
- rewrite Positive.clean_type to perform substitution of {co,contra}-variant only
variables and internal variable beautification at the same time
- For the moment, only call Positive.clean_type at the very end, before returning the result type. Need to investigate if intermediate results would benefit from having less variables.