Liquid Haskell should accept `UnicodeSyntax`

Setting {-# LANGUAGE UnicodeSyntax #-} in a source file allows us to use, e.g., → and ∷ as -> and ::, but this doesn't apply to Liquid Haskell code, which even with UnicodeSyntax enabled still accepts only the latter. It would be nice if we could use UnicodeSyntax in Liquid Haskell as well.

(Given that we can't at the moment, I've had to abandon UnicodeSyntax for my projects that use Liquid Haskell, as having a mix of the two in the same file makes searching and the like more difficult.)