Add explicit this annotation syntax for function types

[goodmind]

Fixes #452
This implements it only for types and declares, doesn't work on statements and expressions

[nmote]

CC @gabelevi

[goodmind]

/cc @gabelevi

[goodmind]

I believe we had conversation with @mroch that it still isn't decided what syntax should be

[goodmind]

I'm probably fine with function foo<this = T>() {} syntax for actual declaration, but not for function types and declare. I think it's impossible to take generic into consideration when doing overloading, is it?
Also this intuitively behaves more like implicit function parameter, rather than generic

[goodmind]

@gabelevi can I close this if you're working on this annotations already?

[Mouvedia]

@gkz can you at least put a reason for the closure?

[gkz]

Sorry, this has been implemented: https://medium.com/flow-type/sound-typing-for-this-in-flow-d62db2af969e