flambda-backend: Use only .ocamlformat* to enable or disable ocamlformat (#1135)

Currently, we have two ways of expressing which files are ocamlformatted:
ocamlformat's own configuration files, and the logic hardcoded into the `make
fmt` implementation. Editors use `.ocamlformat` and friends to see which files
to format, and CI uses `make fmt` to check that things are formatted.
Unfortunately, these mechanisms have gone out of sync - there are several files
that are formatted according to `.ocamlformat` or `.ocamlformat-enable` but are
missing from `make fmt`, so the only effect of enabling ocamlformat on them is a
fun surprise when you go and save a file.

Notably, ocamlformat's behaviour is to take a list of files and format only the
ones it's configured to. So `make fmt` can only go wrong by passing _too few_
files, not too many.

This PR changes `make fmt` so that it just passes every last `.ml` and `.mli`
file in the repo to the `ocamlformat` command line, making `.ocamlformat`,
`.ocamlformat-ignore`, and `.ocamlformat-enable` files the source of truth about
which files get formatted. I've adjusted these files to reflect the status quo,
so the only immediate impact here is that editors behave predictably.

(Note that some changes reflect a subtle fact about ocamlformat: Once a file
is listed in `.ocamlformat-enable`, it cannot be `.ocamlformat-ignore`d. Thus
adding a deeper `.ocamlformat` with `disable=false` is preferable to using a
wildcard match in `.ocamlformat-enable`.)

Author: lukemaurer

lambda/.ocamlformat-enable

@@ -0,0 +1 @@
+disable=true