Implement object type spread (both $Exact and nonexact semantics)

[rosskevin]

Object type spread was introduced in Flow 0.42.0

Documentation is not yet published, but the behavior is documented here and this is referenced in the implementation, as use without $Exact is odd - it makes all prop values from the spread optional.

I have mirrored flow behavior, though I expect the default behavior may change. This too is commented in code so we have some sanity to check and change it in the future.

[rosskevin]

Changes made on this and #2.

[rosskevin]

Trying to get this rebased, but error stack trace line numbers no longer line up...so it's a bit of a pain.

[rosskevin]

@thejameskyle this is rebased and good to go.