Update libasync dependency to 0.8.0

[belka-ew]

libasync 0.7.9 doesn't work with dmd 2.072.0 beta.

0.7.9 causes:

Running ./asynchronous 
object.Error@src/rt/minfo.d(356): Cyclic dependency between module libasync.threads and libasync.posix
libasync.threads* ->
libasync.events ->
libasync.posix* ->
libasync.events ->
libasync.threads*

Program exited with code -11

[dcarp]

Thanks!