`absl::Hex` and `absl::Dec` are now stringifiable through `AbslStringify`. This means that they can be passed to logging, `absl::StrFormat`, `absl::StrCat` and `absl::Substitute`. Note that this change will break unsupported usages of `absl::AlphaNum`. `absl::AlphaNum` is not intended to be instantiated as a stack variable due to lifetime issues. Unsupported usages include:

* constructing an `absl::AlphaNum` as a local variable.
  * For example, `absl::AlphaNum a = absl::Hex(...);`
* declaring an `absl::AlphaNum` as a member variable

Usage of `absl::AlphaNum` as a function parameter will continue to be supported.

PiperOrigin-RevId: 505158534
Change-Id: Idecdf0b3d137482e86b393c7480229343d68eb36