Allow multiline QuickCheck properties.

[kindaro]

Steps to reproduce:

Create a property that spans multiple lines, like this:

-- prop> :{
--      \x -> x == x
-- :}

Expected:

The property will parse and execute correctly, passing the test.

Actual:

### Failure in ... : expression `:{'

<interactive>:106:21: error: parse error on input ‘:’

<interactive>:107:3: error:
    • ‘doctest_prop’ is not in scope at a reify
    • In the untyped splice: $(polyQuickCheck (mkName "doctest_prop"))
Examples: 8  Tried: 8  Errors: 0  Failures: 1

[kindaro]

This is actually a duplicate of #131. I'm sorry. But at least it shows that there's interest.