Fix let bindings

[Chris Warburton]

Looks like `tip` generates broken Isabelle code when it comes to
let-bindings.

This is a known issue on their bug tracker, but it might be easy enough
to work-around rather than spending the effort to fix it.

For the record, here's a valid Isabelle expression (from
https://groups.google.com/forum/#!topic/fa.isabelle/WJWMky_Vt7c ):

  lemma "(let pb = foobar b in f pb (λpad. g (pb * pad :: int))) = bar"
  apply(simp add: mult.commute)

Here's the output we're getting from tip:

  fun foo :: "Baz Bar => Baz Bar" where
  "foo (Quux) = Quux"
  | "foo (fizz local3 local4) = let local2 :: Baz = foobar local3 local4
                                 in fizz local2 (foo (buzz local2 (fizz local3 local4)))"

We should alter the tip output's invalid syntax to be acceptable by Isabelle.