Implicit boxing of large futures causes excessive monomorphization

[osiewicz]

async-task contains an optimisation for handling large futures in the definition of spawn_unchecked. This leads to excessive IR size, as one branch instantiates RawTask with Fut and the other does so with Pin<Box<Fut>>. This probably gets eliminated within LLVM (as the branch itself is trivial), but it's still a bummer that this cannot be truly determined at compile time. I took several stabs recently at getting rid of the unnecessary instantiation, without luck. I do understand why we need the boxing, but it'd be nice to not spend time on generating code we're gonna throw away in LLVM anyways. Getting rid of large-future-boxing reduces the LLVM IR size for my medium-size (~1.5M LLVM IR lines in debug) crate by 7%. This is also replicated in examples from this crate:

Example name	LLVM IR before	LLVM IR after	% of original
spawn	18276	15631	85.52%
spawn-local	39801	34537	86.77%
spawn-on-thread	18667	16031	85.87%
with-metadata	32834	24887	75.79%

Related: https://github.com/rust-lang/rust/issues/85836