Evaluate the layouts for the tasks at compile time

[notgull]

Thank you for the challenge, @michaelwoerister

This pull request adds new internal functionality to async-task that, rather than computing task_layout() at runtime, turns it into an associated const on RawTask. Safety guarantees are still upheld; the program will still abort() at runtime with an invalid task layout.

Note that this bumps up the MSRV to 1.47.0, as that is when the checked_* family of functions were stabilized as const-evalutated. @taiki-e has mentioned that they are comfortable bumping the MSRV to up to 1.48.0, so I hope this shouldn't be an issue.

[notgull]

Note that this may obsolete #29, since the layout is now essentially already cached into each of the VTable functions

[michaelwoerister]

Nice! Great idea to go through an associated const. Thanks for looking into it, @notgull :)

An adapted version of #29 will still be necessary for debuggability (because rustc apparently does not emit debuginfo for associated consts) -- but now we can just put a &'static Self::TASK_LAYOUT reference into TaskVTable instead of storing something in Header.

So, I'd be in favor of merging this PR first.

[taiki-e]

Thanks!