Stabilize the contract macro

[aakamenov]

The existing implementation has some issues and missing features.

Completed:

• Return types in queries for contract_impl are not enforced (#155)
• The macro should allow any Result<T, E> return type (#155)
• Support reply entry points (#161)

Outstanding:

Support for sudo entry point

New entry point in CW 1.0 Needs more research into its specific behaviour. It might be the case that they require additional features to allow for customization. Should be relatively straightforward to add support for that otherwise.

[aakamenov]

With the complete rework of the macro into Fadroma DSL this can now be considered more or less complete. There only remains the support for sudo which has now been moved to #162