REPL let bindings can't define infix operators

[brianhuffman]

I can define named values or functions at the REPL with let, but I can't define infix operators:

Cryptol> let f x y = x && y
Cryptol> let x & y = x && y

Parse error at <interactive>:1:7,
  unexpected: &
Cryptol> let (&) = (&&)

Parse error at <interactive>:1:6,
  unexpected: &
Cryptol> let (&) x y = x && y

Parse error at <interactive>:1:6,
  unexpected: &

I would expect all of those forms to work.

[robdockins]

REPL type synonyms would also be convenient

[robdockins]

Fixed via #1255