Add: CompactCowStr

[aobatact]

This pull request aims to add a new struct called CompactCowStr to incorporate Cow for CompactString.
This proposal has been previously discussed in issue #223.

Question

• Reuse of Repr:
  CompactCowStr reuses Repr, which may potentially impact CompactString.
  If Cow flag of LastUtf8Char leaked to CompactString, this might cause UB.

• Methods:
  Which methods equivalent to those in CompactString are necessary for CompactCowStr?

• Inlining:
  For the new method, should we inline it?

• Name Discussion:
  Previous pull request suggested the name CompactCow, but considering its relationship to String, I have changed it to CompactCowStr.

[aobatact]

miri is failing with "unknown unstable option: miri-check-number-validity".
It seems the CI needs to be fixed?

[NobodyXu]

I think that miri cmdline option might be removed in the nightly, so it probably needs to also be removed from CI?

[aobatact]

Fixed CI in #373

[ParkMyCar]

Hey @aobatact, sorry for the delay and thank you for the PR! I took a quick look and I think we're on the right path, but I've paged out a decent amount of the context here. I have a long train ride this weekend which I'll use to dive in deep here.

[jf2048]

Is there some reason this needs to reuse Repr with its own niche? A simple enum implementation seems to have the same layout.

pub enum CompactCowStr<'a> {
    Borrowed(&'a str),
    Owned(CompactString),
}
assert_eq!(size_of::<CompactCowStr>(), size_of::<CompactString>()); // true
assert_eq!(size_of::<Option<CompactCowStr>>(), size_of::<CompactString>()); // true