Add TokensByOwner for cw721-base

[ethanfrey]

Closes #81

Builds on #108 (merge that first)

Ports to use cw-storage-plus rather than cosmwasm-storage, then makes use of IndexedMap

• Port to cw-storage-plus
• Better support for &str Map keys
• Use IndexedMap
• Enable query by owner
• Cover query by owner with tests
• Allow setting bounds (pagination)
• Raise issues for remaining TODOs

[maurolacy]

Looks good. I think the use of bounds is still a little bit confusing / needlessly complex.

Added some comments to try and help clarifying the design.

[ethanfrey]

My takeaways. Only use one type:

pub enum Bound {
  Inclusive(Vec<u8>),
  Exclusive(Vec<u8>)
}

and accept Option<Bound> in range.

This will lead to lines like:

`let start: Option = start_after.map(|human| Ok(Bound::Exclusive(deps.api.canonical_addr(&human)?)).transpose()?;

Which is not so legible or user-friendly. I guess having a function for handling the human_addr case is good. For a string/int, we could get:

let start: Option<Bound> = start_after.map(|s| Bound::Exclusive(s.into()))

or

let start: Option<Bound> = start_after.map(|i| Bound::Exclusive(i.to_be_bytes().into()))

both of which are simple enough, without all these helpers.

---

Does the above make sense to you? Other suggestion?

[maurolacy]

Makes sense. Will review later again with the IDE, to see and try some variants.

[maurolacy]

I understand this is WIP, but what I see looks good to me.

Powerful stuff here, and in the storage-plus.