Makes the following changes:
 1. Generalises the type signatures of some functions relating to alternatives
    so that the type of "variables" and "expression" is not specified
 2. Puts the bulk of the alternative-filtering code into a new function filterAlts
    (in CoreUtils) that can be used outside of the SimplM monad
 3. Allows prepareAlts to return a null alternatives list if none are applicable -
    it turns out that this case was already handled by the caller (in the simplifier).
    This should result in a modest optimisation improvement in some cases.

Conflicts:

	compiler/coreSyn/CoreUtils.lhs
	compiler/simplCore/SimplUtils.lhs