Use core::error

[robertbastian]

Fixes #5964

The only remaining uses of std are:

• DataError::Io and DataError::with_path
  • with_path can be replaced by with_display_context(path.display()), but the std::io::ErrorKind is not in core (it could be, it's a fieldless enum).
• icu_provider::log falling back to println! (negative logging feature)
• icu_capi keeps its std feature, as that's required to build a static library with panic=unwind. I'm not sure what the use case for this is, but we use it as a baseline in the size benchmark.

[Manishearth]

All three of those cases seem fine by me.

I would also be okay with erasing the Io error context (Io errors rarely contain enough information _anyway)) in DataError, btu we don't really need to so let's leave it as is.

[Manishearth]

Is there a reason this is not "fixes #5964"?