FTL: Yield futures without overhead

ftl::yield, which lifts T to std::future<T>, incurs the cost of
allocating, ref counting, and locking the latter's shared state.

Consolidate the existing std::future extensions into ftl::Future,
and optimize ftl::yield by including static storage for T within.

Bug: 232436803
Test: simpleperf (-31% cycles in postFramebuffer)
Change-Id: I9a7ca7de17e7af10515de97d2f6a0dfa24e35d7a