Added entry_points! macro to reduce boilerplate when writing contracts

[reuvenpo]

I wrote this macro to allow us to reduce boilerplate when writing our own contracts, and figured it would be a good idea to suggest it upstream.

It's not the most flexible macro ever (and trust me, I know how to make macros flexible) but i had trouble taking the paths to the implementation functions or even just to the contract module itself in a flexible way because of a weird limitation of macro rules.

I added a doc-comment to the macro definition, and replaced the boilerplate in the example contracts to showcase the macro.

To illustrate, hackatom's lib.rs file now looks like this:

pub mod contract;

#[cfg(target_arch = "wasm32")]
cosmwasm_std::entry_points_with_migration!(contract);

Update: looks like I can't always require migrate to exist. I'll try to make that work.
Update: Done.

[webmaster128]

Very good

[ethanfrey]

Looks good to me.

Can you update MIGRATING.md and make a PR on cosmwasm-template with this change? (Actually we need a whole 0.9 branch there, so maybe the comswasm-template stuff can come later)

[reuvenpo]

Sooo... Should I do any of those things or not?

[webmaster128]

Sooo... Should I do any of those things or not?

No, thanks. I'll take care of that.