Ref var improvements

Add some new error messages for ref vars:

Fatal error when using ref vars as members of classes/records.
Fatal error when ref vars are declared, but not initalized.
Fatal error when assigning a non-const ref to a const ref formal.

Update some ref var tests

Add two new futures:
test/variables/kbrady/ref-in-tuple.future
test/variables/kbrady/ref-member.future

Retire three old futures:
test/variables/sungeun/refvar-compare-nil.future
test/variables/sungeun/refvar-no-init-with-def.future
test/variables/sungeun/refvar-no-init-with-use.future
test/variables/kbrady/ref-to-const.chpl checks this one.