Prefix MAFuture symbol names with KW_

This basically treats MAFuture as an internal implementation detail of
Kiwi.

If an app using Kiwi for testing itself includes MAFuture, there likely
won't be duplicate symbol errors at compile time as Kiwi will be
compiled into the test bundle, and then injected at runtime. Prefixing
MAFuture symbols with KW_ mitigates the risk of undefined behaviour in
this scenario.