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.

[gpetiot]

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