opam-fmt design

Need: we want the opam file commited in the opam-repository to be
formatted such that programmatically parsing and re-printing them is
a no-op. Today we don't have this, and this means that bulk script
changes to the opam-repository metadata creates much larger diff than
necessary.

In fact, we propose to install many different scripts named
  opam-fmt-$OPAMVERSION
with each script depending on the corresponding version of opam-lib.

Then, we would have an
  opam-fmt
script that would default to the script corresponding to the current
OPAM version.

Implementation detail: opam-fmt-$OPAMVERSION should refuse to process
opam files whose opam-version field is larger than $OPAMVERSION. It
should always produce output with an opam-version field equal to
$OPAMVERSION. This means in particular that using opam-fmt on an
opam file may silently update its opam-version field.

Versioning: opam-fmt-$OPAM_VERSION.$OPAMFMT_VERSION_NUMBER

Features:

  opam fmt --version=$OPAMVER $ARGS
    calls opam-fmt-$OPAMVER $ARGS
    (by default, the current opam version)

  opam fmt-$VER <file>
    reads the opam file with opam-lib's parsing functions, and
    re-print them on stdout with opam-lib's printing functions

  opam fmt-$VER --stdin
    reads the opam file on standard input

  opam fmt-$VER [<file>|--stdin] --output <output-file>
  opam fmt-$VER <file> --inplace

  opam fmt-$VER --version
    returns the OPAM version

  opam fmt-$VER [<file>|--stdin] --check
    fails with an error message if opam-fmt output would differ from
    its input

    to be used in opam-repository automatic QA

  opam fmt-$VER
    print usage message

Note in particular that
  opam fmt --version=1.2 --version
is valid and prints 1.2