wip: feat(scrt-contract): nice bindings

[egasimus]

This is meant to allow for contracts to be more easily tested in a browser. Currently when you load a WASM blob in a browser, you just get the entrypoints that are meant for cosmwasm-vm, which only take 1 or 2 u32 pointers. By identifying the correct API layers to hook into, something similar to the schema-based API wrappers of scrt-contract can be generated as a feature of the contract itself (behind a feature flag), exposing the "nice" API directly.

See also: scrt-harness as another step towards struct Contract.

[egasimus]

• It is now possible to test the business logic of a Fadroma-based smart contract in the rich interactive environment of your garden variety Web browser (provided you do your own serialization on the JS side with TextEncoder/TextDecoder). Example here.
• The source of scrt-contract module was refactored into three layers - the entrypoint bindings, the API types, and the API implementation.
• Considering deprecation of State section as it feels unnecessarily bolted-on.
• First appearance of a struct Contract { deps, env } - this and scrt-harness should be enough to come up with some future simplifications to the contract! macro which to hopefully give scrt-contract a slightly better onramp profile :-)