Revamped concurrency API

[Jeremie Pelletier]

Jeremie Pelletier wrote:
> I agree here, while I think Andrei's book is a must for the community to 
> grow, it should rush the compiler. I strongly believe D2 needs proper 
> shared and lent semantics, or else we're just creating a stepping stone 
> for D3, leaving D2 in a state that isn't suited for concurrent model 
> implementations in a type-safe manner.

s/should rush/shouldn't rush/