Attempt Polymorphization

[james7132]

Partially fixes #78.

This attempts to minimize the amount of monomorphized codgen created while using this crate.

• Header<M> has been split into Header and HeaderWithMetadata<M>, using repr(C) to keep pointers trivially castable between each.
• Move any header-only operations onto the Header type.
• Move out multiple entries of the TaskVtable to operate without every generic argument on RawTask, ,just the ones they need.
• Make functions like get_output a function on the vtable itself instead of put in the vtable, both avoids an extra dynamic dispatch call, and shrinks the vtable.
• Embed the RawWakerVtable in the TaskVtable and pass it into polymorphized functions.
• Use #[inline(never)] on these polymorphized functions.

This PR also shifts to use byte_add, which does bump the MSRV to 1.75. If this is too much of a bump for such a gain, I can revert that.

Benchmarks:

master
test task_create ... bench:         131.02 ns/iter (+/- 98.60)
test task_run    ... bench:         149.03 ns/iter (+/- 106.22)

polymorphization
test task_create ... bench:         123.16 ns/iter (+/- 33.41)
test task_run    ... bench:         150.04 ns/iter (+/- 21.27)