Support zero-copy in UnicodeSet

[sffc]

We should figure out the right way to make UnicodeSet support zero-copy deserialization. I propose that we change UnicodeSet itself to wrap a ZeroVec, thereby gaining a lifetime parameter.

[sffc]

Discussion:

• @iainireland - How does the lifetime parameter work over FFI?
• @Manishearth - In JavaScript we can do clever things involving the GC; in C++ you have to be careful. And you can always set the lifetime to 'static.
• @sffc - What is the return value of Unicode property getters like get_alnum_property?
• @Manishearth - Could it have 's? Or maybe a lifetime from the DataProvider?
• @sffc - It could return a DataPayload.
• @iainireland - As we work on the return type, does it affect what shows up on the C++ side of FFI?
• @Manishearth - Instead of boxing a UnicodeSet, it would box a DataPayload. So on the C++ side, you wouldn't need to care, but the FFI layer would need to be updated.
• @iainireland - Does this affect UnicodeSetBuilder or only UnicodeSet?
• @sffc - Only UnicodeSet.

Conclusion:

• UnicodeSet should wrap ZeroVec
• The Unicode property getters should return DataPayload

[echeran]

In the process of applying feedback changes, I made a minimal repro case for what seems to be a compiler bug (filed at: rust-lang/rust#87932)