Implement choice type

[iliclaey]

Fixes #114

The choice type determines if a string is in a list of possible choices (name from to the python optparse choice option). This would replace the current heavy usage of string with match(SELF, '^(a|b|c)$') with

    "attr" ? choice('a', 'b', 'c')

String as a basetype is the most common one (or even only one). makes no sense for boolean i think, and long already has long(1..4) which is something similar. so only float is not covered, which i think is acceptable.

[stdweird]

it's true that choice with one argument makes not much sense, but it should be possible (i assume it is). re rephrase this to one or more

[jrha]

This title of this PR should be imperative, i.e. "Implement choice type", copying some of the description from the linked issue would also be helpful to people reading change logs later on.