micro performance improvement

Conflicts:
	types/types.ml
	types/types.mli

and Cap (to fix)

  to detect that we have the most precise solution
* Apply clean_type more agressively but on smaller types, for each of the partial solutions
  rather than on the (big) intersection of all the solutions.
* Small optimisation in substitutefree: do not do anything if the type has no variables

- 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.

Conflicts:
	tests/typed/src/main.ml
	tests/typed/src/printer.ml
	tests/typed/src/printer.mli

Conflicts:
	tests/libtest/tallyingTest.ml
	types/types.ml
	typing/typer.ml

Do not call Arrow.apply since the types bound to the variable Gamma will be the result of the application.

- fix typos in comments
- indent code
- move the Positive module further down types.ml, so that there is no
need for forward references when we implement substitutions over record types.