Fixes for fb tests.

Created this to address #62 . It should also address #61 

Added `mseq` in `consWhile`, which removed the pattern match issue.

Also, dead casts override other casts when necessary, given that we don't support multiple casts on the same expression.

About removing the bounds check in `body` in: 
https://github.com/UCSD-PL/RefScript/blob/fb_fixes/tests/pos/fb/min-index-01.ts

I can't seem to get K-var inference to work (i.e. not have to spell out the refinement for `reduce`)