v0.4.0 - Better Storage

This release mostly includes the work of @srdtrk in #53. Thanks Srdtrk!

It revamps the secret-toolkit-storage package to make it more similar to cw-storage-plus and much easier
to use. It also removes the Cashmap type from the incubator in favor of KeyMap in secret-toolkit-storage.

This is a summary of the changes and additions in this release:

• Minimum Rust version is bumped to the latest v1.63. This is because we want to use Mutex::new in a const fn.
• No more distinction between Readonly* and *Mut types. Instead, methods take references or mutable references to the storage every time.
• Usage of PrefixedStore is made mostly unnecessary.
• Storage type's constructors are const functions, which means they can be initialized as global static variables.
• Added secret-toolkit::storage::Item which is similar to Item from cw-storage-plus or TypedStore from cosmwasm_storage v0.10.
• Added secret-toolkit::storage::KeyMap which is similar to Cashmap.
• Cashmap is completely removed.

A full guide to using the new storage types can be found
in the package's readme file.