Distinguish between ternary's : and arrow fn's return type

[nicolo-ribaudo]

Q	A
Bug fix?	yes
Breaking change?	no
New feature?	no
Deprecations?	no
Spec compliancy?	no
Tests added/pass?	yes
Fixed tickets	#58, #429
License	MIT

@hzoo This is #573 but without flow errors. Apart from 5f176cb, this code has already been reviewed.

[nicolo-ribaudo]

I rebased to resolve the merge conflicts. I squashed the commits because, since I wrote an implementation of the logic to avoid throwing errors for parenthesized expressions which are not valid parameters and then totally rewrote it, I would have needed to resolve the conflicts for a lot of unuseful commits.

The original commits can be found at https://github.com/nicolo-ribaudo/babylon/tree/issue-58-ternary-arrow-flow-copy

[nicolo-ribaudo]

I just pushed some minor changes (fixed three new flow errors and removed some duplicated code)