Drop MSRV to 1.63 w/o `RUSTC_BOOTSTRAP` hacks with a bad assumption

Drop MSRV to 1.63 w/o `RUSTC_BOOTSTRAP` hacks with a bad assumption

While the Rust language reference says "you should not rely on
this", in practice slices are laid out in memory as a two-tuple of
`(pointer, length)`. Here we rely on that assumption to replace
`alloc::slice::from_raw_parts` with a `core::mem::transmute`.