Fixes jbaldwin#109 fn overload on task promise::return_void bug (jbal…

…dwin#110)

The return_void() overload was throwing, but this is
incorrect per the standard, it should effectively do
nothing.  This was an error on my part since I over-
loaded the non-void task<T> return_value(T) -> void
and return_value() -> T fns.  This overload isn't
possible with the void versions since the signature
is identical, thus the bug caused the compiler's coroutine
version to re-throw exceptions at the wrong time.

Update docs on io_scheduler for inline processing (jbaldwin#84)

* Update docs on io_scheduler for inline processing

Support gcc 10.3.1 (fedora 33 updated)
Update ci.yml to run fedora 32,33,34 and support both
gcc 10.2.1 and 10.3.1

* fedora 32 -> gcc-c++ drop version

* Update ci.yml and test_latch.cpp

coro::mutex uses function for unlocked state to reduce memory (jbaldw…

…in#77)