Fix lifetime of MultiIndex/UniqueIndex to be able to accept &str

[ethanfrey]

There is an index function that takes K: PrimaryKey => Vec<u8> more or less. And indexes wrap this allowing a number of functions.

Currently this only works for static lifetimes (eg. PkOwned, String). It should be possible to support slices (&str, &[u8]) which will not just avoid a copy, but make the usability much nicer, as many types auto-deref to one of those slices.

Investigate what is needed to make this work for UniqueIndex (the simpler case), and if you can get a Proof of Concept to compile, then we decide if it makes sense to continue on this path.

[ethanfrey]

let's try to fix this if we can. if it is too painful, let's close it and accept the status quo.
but good to stabilize the index functions

[maurolacy]

Will take another stab at this.

[ethanfrey]

This seems like a huge amount of work for marginal return. Closing for now.