Prepare for strict provenance in the Default implementations.

[V0ldek]

Strict provenance is an attempt on bringing sanity to pointers. It is tracked here. We are already enabling Miri's -Zmiri-strict-provenance flag to catch mistakes, which is pretty useful, since the API dabbles with raw pointers. It caught the Default bug in the first place #11 .

Miri checks now fail, since the fix for #11 introduced an integer-to-pointer cast in https://github.com/V0ldek/aligners/blob/main/src/bytes.rs#L264. This should be relatively easy to fix -- just use conditional compilation with cfg to see if the strict_provenance feature is enabled, and if yes use the new API. When SP is stabilised, this will be the new implementation anyway.

This is the only place in which we do magic with raw integers, since it's a zero-sized allocation, which AFAIK has to be magic.

[V0ldek]

Actually, the polyfill crate might be an easier change: https://crates.io/crates/sptr