Decide on the fate of the Borrowed variant in DataPayload

[sffc]

The Borrowed variant of DataPayload[Inner] pulls in the 'd lifetime parameter. This has caused some challenges, including:

• Less ergonomic (extra lifetime parameter)
• Issues related to trait objects (see comment in erased.rs)

The main use case for the Borrowed variant is to pave the way toward future no-alloc use cases.

If we decided to keep the Borrowed variant, there are some options we could explore to work around the trait object issues mentioned above.

Also decide on the names of the lifetimes ('d and 's).

[sffc]

2021-06-04: Conclusion: Removed the Borrowed variant, and add it back in the future when we have a clearer use case.

[sffc]

• @echeran - I think 'cart and 'data are short and descriptive
• @Manishearth - I think the cart should not be something most users worry about
• @sffc - The issue with 'data is that it's unclear what happens if we need to add a second lifetime
• @Manishearth - Okay, that's convincing

Conclusion: Eliminate 'd and replace 's with 'cart

[sffc]

Some more ideas for the lifetime name:

• 'ds for "data struct"
• 'st short for "string" or "struct"
• 'ld for "locale data"
• 'locd for "locale data"
• 'icudata
• 'data

[sffc]

2021-06-10: Go with 'data since it is more clear when reading code.

[sffc]

Check if impl_dyn_clone is still needed.