AddClause does not not generate a correct clause for operators

[petithug]

When using AddClause on an interface implementation, the operator clauses are invalid.

See also #248 and #247

Steps to Reproduce

Loading the following in the REPL and executing ":ac 4 Eq"

data Blah : Type where
  MkBlah : Blah

Eq Blah where

Expected Behavior

data Blah : Type where
  MkBlah : Blah

Eq Blah where
  (/=) x y = ?something_rhs

Observed Behavior

data Blah : Type where
  MkBlah : Blah

Eq Blah where
/= x y = ?/=_rhs