Redesign Regex constraint

[Olian04]

The Test constraint should always assert true and then return the regex context again, this way multiple tests can be chained.

Current design	Redesign
R.Regex.Test(str).True	R.Regex.Test(str)
R.Regex.Test(str).False	R.Regex.not.Test(str)
R.and([R.Regex.Test(str1).True, R.Regex.Test(str2).False])	R.Regex.Test(str1).not.Test(str2)

Pros:

• Can chain tests

Cons:

• Less clear whats going on