breaking: remove cancel, replace with destroy and quit

[shawnl]

#1363 discusses current rase conditions in std.event.loop. These race conditions can be addressed in user-space, however cancel is too feature-full and has introduces a race condition of its own.

This proposal is to return to what llvm gives us and remove cancel for llvm's destroy

suspend will no longer be responsible for running the defer and errdefers, and instead these will be run by destroy and quit. quit is just shorthand for destroy @handle().

It will cause undefined behavior to call destroy on a running co-routine, but in the NM scheduling model a co-routine needs to be able to destroy itself with quit: https://bugs.llvm.org/show_bug.cgi?id=38805