clean up MaudeHelper

Make the never-modified basicSorts and constants sets private
to MaudeHelper and expose only isBasicSort and isConstant methods.

Move the mutable separators collection into MaudeFilter, which
is the only place it is used.
Make Maude list terminators from label also

This applies the earlier change to list terminators
in the Maude backend as well.
Merge pull request #931 from bmmoore/list-terminators-maude

Maude list terminators from label also