Replace usage of `mem::uninitialized` with `MaybeUninit`

[memoryruins]

When the following comment was added in #1845, hyper's minimum supported rust version was 1.27.0

hyper/src/proto/h1/role.rs

Lines 1 to 3 in 4e9a006

	// `mem::uninitialized` replaced with `mem::MaybeUninit`,
	// can't upgrade yet
	#![allow(deprecated)]

which I assume is why std::mem::MaybeUninit, stabilized in 1.36.0, could not be used yet. Today, hyper's MSRV is 1.46.0, so it should now be possible to investigate removing the #![allow(deprecated)].

[seanmonstar]

Good call!

[mohsen-alizadeh]

Refactored the code.

[Soveu]

One problem with refactoring the code is that httparse::Request::new accepts &mut [Header], so it is assumed that they are initialized. It would need to accept &mut [MaybeUninit<Header>]

Otherwise MaybeUninit::uninit().assume_init() (like it is done here) gives the same problems as mem::uninitialized

[RalfJung]

FYI, this seems to be one of the major sources of regressions in rust-lang/rust#87041.