Second Draft: Coroutines

[wesleycolton238]

Thanks for clarifying that explicit and implicit yielding can 
coexist depending on the library type. A small side-by-side 
example would help make that distinction easier to follow: the 
same operation using explicit await and implicit yielding, with 
each possible suspension point marked. Showing which parts the 
compiler generates and which the library implements would also 
address the questions about their separate responsibilities. That 
would give readers a concrete example to refer to when evaluating 
the proposed syntax.