Generate COMPLETE pragma for pattern synonyms

[fizruk]

The COMPLETE pragma ensures the compiler that a certain combination of patterns in exhaustive.

For example:

data TermSig scope term = AppSig term term | LamSig scope | LetSig term scope
mkPatternSynonyms ''TermSig

will generate

pattern App t1 t2 = Node (AppSig t1 t2)
pattern Lam x body = Node (LamSig (ScopedAST x body))
pattern Let t1 x t2 = Node (LetSig t1 (ScopedAST x t2))
{-# COMPLETE Var, App, Lam, Let #-}