Deprecating "features"

[emillon]

The "features" field is accepted in opam files:

opam/src/format/opamFile.mli

Lines 457 to 458 in 4d9db69

	(** Contents of the 'features' field *)
	val features: t -> (OpamVariable.t * filtered_formula * string) list

It is documented as an experiment but it looks like the feature was never used. Something similar is likely to be useful with package variables but it looks like this first implementation can be deprecated in 2.1 or 2.2.

[dra27]

See #4519 (comment)

[AltGr]

This is different from libraries and syntax in that, although unused, it is documented and doesn't do absolutely nothing.
I'd say we can keep it (with the experimental flag) until we have an actual implementation for package variables.