Remove Async impls for std types except for Result and ()
Remove Async impls for std types except for Result and ()

Keep the impl for Result because it's the primary implementation people
are expected to use, and keep () as a trade-off for convience.

As discussed in #10.

posix4e/eventual,posix4e/eventual,tempbottle/eventual,carllerche/eventual,sisirkoppaka/eventual,tempbottle/eventual,seanmonstar/eventual,mlalic/eventual,carllerche/eventual,blaenk/eventual,benaryorg/eventual,benaryorg/eventual,seanmonstar/eventual,blaenk/eventual,mlalic/eventual,sisirkoppaka/eventual