Remove lwt_ppx dependency

[n-osborne]

This PR proposes to get rid of the %lwt syntax from lwt_ppx in favor of the
Lwt_syntax.

It is based on #814, please consider only the last commit.

This is join work with @Julow (who wrote the tool making the automatic changes)

Most of the changes has been done automatically using the prototype tool
here. This is the reason why
the PR is one big commit.

This is a first step toward moving from Lwt to effect-based concurrency. The
idea is to homogenize the syntax in order to have less cases to handle (again
by an automatic tool).

This is also an independent contribution, as it removes one dependency and one
preprocessor. Which can be considered a win by itself.