Added new async file copy example.

Fixed directory_entry causing miscompilation on GCC and clang.

Reworked when_all(ops) to reuse more code.

Added steady_clock to chrono mappings.

test_all now disables headers only on Mingw, as Mingw couldn't cope with the binary size.
Had accidentally put -Wno-mismatched-tags into GCC instead of clang.
Updated docs.
Fixed link breakage on mingw.