Support life-timed references in UniqueIndex and MultiIndex keys

[maurolacy]

We're currently using PkOwned for MultiIndex and UniqueIndex keys. See also related #199, for another attempt to remove PkOwned.

This is what we get when changing the name MultiIndex to use &[u8] instead of PkOwned (in the indexed_map.rs tests):

   Compiling cw-storage-plus v0.5.0 (/usr2/home/mauro/work/cosmwasm-plus/packages/storage-plus)
error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
   --> packages/storage-plus/src/indexed_map.rs:209:50
    |
209 |             name: MultiIndex::new(|d, k| (d.name.as_ref(), &*k), "data", "data__name"),
    |                                                  ^^^^^^
    |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the body at 209:35...
   --> packages/storage-plus/src/indexed_map.rs:209:35
    |
209 |             name: MultiIndex::new(|d, k| (d.name.as_ref(), &*k), "data", "data__name"),
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...so that reference does not outlive borrowed content
   --> packages/storage-plus/src/indexed_map.rs:209:43
    |
209 |             name: MultiIndex::new(|d, k| (d.name.as_ref(), &*k), "data", "data__name"),
    |                                           ^^^^^^
note: but, the lifetime must be valid for the lifetime `'a` as defined on the function body at 207:18...
   --> packages/storage-plus/src/indexed_map.rs:207:18
    |
207 |     fn build_map<'a>() -> IndexedMap<'a, &'a [u8], Data, DataIndexes<'a>> {
    |                  ^^
note: ...so that the expression is assignable
   --> packages/storage-plus/src/indexed_map.rs:216:9
    |
216 |         IndexedMap::new("data", indexes)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: expected `indexed_map::IndexedMap<'a, &'a [u8], _, DataIndexes<'a>>`
               found `indexed_map::IndexedMap<'_, &[u8], _, DataIndexes<'_>>`

error: aborting due to previous error

Analyze if we can use &[u8] for keys / subkeys in UniqueIndex and MultiIndex, after resolving these lifetime conflicts.
As a reference, Map allows using &[u8] for keys without issues.

[maurolacy]

Closing as a duplicate of #232.