Pretty printer for Cryptol.Parser.AST.Expr parenthesizes infix operators.

(The printer for Cryptol.TypeCheck.AST.Expr already does this.)

Previously:
Cryptol> :t (+)
+ : {a} (Arith a) => a -> a -> a

Now:
Cryptol> :t (+)
(+) : {a} (Arith a) => a -> a -> a