Keep Storage external to Bucket

[ethanfrey]

Inspired by #557 but also a long struggle with the Bucket and ReadOnlyBucket types.

We can make the Bucket always immutable, just by storing the access/mapping info there. And then pass in &Storage or &mut Storage everywhere. We can use Bucket or &Bucket everywhere, also in queries, and the compiler will automatically prevent use from calling the mutable functions if we only have immutable storage.

This also let's us have 2 buckets open at once without worrying about lifetimes.

The downside is one more argument to each call. However, as the typical usage is more or less:

let acct = tokens(&deps.storage).load(&sender_raw)?

we just transmute that to:

let acct = tokens().load(&deps.storage, &sender_raw)?

tokens (the bucket constructor with type and namespace info) need no more arguments, and could theoretically be a const (created compile time) if we migrate from Vec<u8> to &'a [u8] for namespace storage, which I have done in #557, making these even shorter to write and faster to run.

You can look at a202e4d to see what it took to migrate a moderate contract to the new form (not much).

• move Storage into arg for Bucket
• also update Singleton likewise.
• remove unused TypedStorage
• Update CHANGELOG, MIGRATING
• Update README

[ethanfrey]

After discussion, I will not continue this in cosmwasm, and not modify Bucket, but build the new structures with new names in cosmwasm-plus. We can extend cosmwasm-storage later when they are stable, but for now, let's keep it stable.

Leaving this PR until all logic has been moved to cosmwasm-plus

[ethanfrey]

This has been done in https://github.com/CosmWasm/cosmwasm-plus in the cw-storage-plus package (see linked PR)