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
I'm not clear on why this if (!init) check is nested within a if (!varlocation) conditional rather than appearing at the top of this if (var->hasFlag(FLAG_REF_VAR)) conditional.  It seems that if init is NULL, we will fall through the first "FLAG_CONST" conditional since toSymExpr(init) will return NULL and that therefore varLocation will also be NULL.  In which case it seems less confusing/convoluted just to assert that there's an initializer right off the bat (rather than requiring someone to untangle the logic as I just did).  But am I missing something subtle?