regex type should be dropped

[Gustavo Niemeyer]

There are issues with the current regex type:

1) The type of the value is not a regex, it's a string. regex is what
we use to validate the actual value. Once we reintroduce the feature,
"validator" should be a field of the "string" type.

2) It's currently binding the metadata of Ensemble to the Python
language. Python is the only language with this particular format for
regexes, and it's a wrong choice to be binding it that way. Once we
reintroduce something like this, it'd be better to have the format
being a more widely spread one, such as POSIX extended or similar.

With those in mind, I suggest dropping it for now, and when we bring
it back we can simply add a new "validator" field to the "string"
type instead.