Represent the expr sequence as a list

[gpetiot]

Not urgent, but it cleans up a lot of stuff in sugar/ast about sequences.

The diff is caused by:

• comments attached after the sequence were moved to after the last element, I'm not sure we should continue to do that, to me the diff looks better on this
• the special case added by Josh about adding a break before the semicolon that follows a debug/trace call, this has been factorized with the rest, with a match on the extension name. Not sure how much people would complain if we removed it? We can also only apply that for the profile = ocamlformat case, or just remove it.

[Julow]

I love this! This removes a lot of tricky code.

the special case added by Josh about adding a break before the semicolon that follows a debug/trace call, this has been factorized with the rest, with a match on the extension name.

It would be nice to have a test case for this. I think it's fair to remove it otherwise.

test-branch finds a similar case in irmin but with [%log.debug], the semicolon was broken before so it seems that the rule was more general.

Also, this might remove some rewriting that were done before, like removing parentheses around sub sequences, rewriting extensions into a shorter form. It would be nice to show the change in the tests.

[gpetiot]

There is already a test of the facebook trace calls: test/passing/tests/pre_post_extensions.ml