Look in data definition when checking dangerous vars

[benl]

data TwoThings r1 r2 r3 a 
      = ThingOne (Int r2)
      | ThingTwo (Maybe r3 b)

wrt the mutability / backtracking problem.

The rule should be at generalisation time, If a region var makes other variables dangerous, then hold it constant unless it already has a mutability constraint. If the user wants it to be mutable, then require them to supply a type signature. Otherwise leave it constant.

For example, if we are to generalise:

 foo :: TwoThings r1 r2 r3

then make r1 and r3 constant, but leave r2 unconstrained.