Refactors logic of poll::AsyncSink::is_[not_]ready()
Refactors logic of poll::AsyncSink::is_[not_]ready()

Expressions are easier to understand when
negations are reduced to the bare minimum.

This commit moves the actual logic to `is_ready()`.
`is_not_ready()` is now a simple negation of `is_ready()`.

alexcrichton/futures-rs,Idolf/futures-rs