Add tcast crate with TransparentCast derive

[sffc]

Fixes #7607
Replaces #5101
See #6915

I was trying to use ref_cast, but found that it had multiple deficiencies:

1. Doesn't support standard pointer wrappers like Box
2. Implements a trait by default, which can violate field invariants
   • Note: it supports a private conversion function via RefCastCustom
3. Not highly suited for use as a dev-dependency only

So, I made my own proc macro, TransparentCast.

The use cases are the same as RefCast, as listed in #7607, but it resolves the deficiencies listed above.

I am as sensitive to dependency bloat as everyone else, so I specifically designed this derive to be usable as a dev-dependency. See the example. However, this PR is not intended to be the place to debate whether we decide to use that strategy in icu4x.