Implements alias annotation and allow multiple deserialization renames.

[Lymia]

This implements #1456 and #595.

Some things in the implementation I'm not sure of:

• This is only implement for fields and enum variants. While it'd make sense on containers for some formats like RON, it doesn't seem like the API actually allows for this.
• The current implementation does not reflect the fact that it accepts aliases in the VARIANTS/FIELDS arrays passed to deserialize_struct, etc. I believe this is the right behavior because encoders like bincode care about the length of this list, and the flatten attribute causes field names that don't appear in this list anyway, but I can't be 100% sure.

[dtolnay]

Thanks! This is terrific. Very nicely done.

[dtolnay]

Released in serde_derive 1.0.85.