Add State derive macro

[mappum]

Closes #41

Description:
Adds the updated version of what was formerly the #[state] attribute macro, to allow structs to easily compose children which themselves implement the State trait. Most application-layer development will simply consume APIs from lower-level State implementations, adding methods on top or just exposing the fields.

type Address = [u8; 32];

#[derive(Encode, Decode)]
struct Account {
  balance: u64,
  nonce: u64.
  pubkey: [u8; 32],
}

#[derive(State)]
struct MyState {
  accounts: Map<Address, Account>,
  total_coins: u64,
}

TODO:

• Support named structs
• Support tuple structs
• Support enums (derive(State) for enums #27)
• Support generic parameters (derive(State) doesn't support structs with generic parameters #32)
• Tests

[codecov-commenter]

Codecov Report

Merging #42 (9cc9de0) into develop (7da16ec) will increase coverage by 0.60%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #42      +/-   ##
===========================================
+ Coverage    64.53%   65.13%   +0.60%     
===========================================
  Files           14       14              
  Lines          750      763      +13     
===========================================
+ Hits           484      497      +13     
  Misses         266      266              

Impacted Files	Coverage Δ
src/state/mod.rs	100.00% <100.00%> (ø)
tests/derive.rs	100.00% <100.00%> (ø)

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