Addr ref helper

[ethanfrey]

Closes #868
Alternative to #869 (close that if we take this)

This solves the AddrRef need for use in cosmwasm-plus contracts and includes nice helpers for conversion between Addr and AddrRef.

AddrRef::unchecked is a const fn, so we can use like:

const foo = AddrRef::unchecked("foo");
call_with_addr(foo.into());

[ethanfrey]

There are better solutions to this "problem". See CosmWasm/cw-plus#264 and CosmWasm/cw-plus#265 and it belongs in storage-plus anyway.