Provides the macros needed to write fvm contracts

## Basic Usage

Macros can be used in contract for simplify contract preparation process，
like this:

 ``` rust
 use fvm_macros::contract;
 use fvm_macros::storage;
 use fvm_std::collections::hyper_map::HyperMap;

 #[storage]
 pub struct SetHash {
     map: HyperMap<String, String>,
 }

 #[contract]
 impl SetHash {
     fn new() -> Self {
         Self { map: HyperMap::new() }
     }

     pub fn set_hash(&mut self, key: String, value: String) {
         self.map.insert(key, value);
     }

     pub fn get_hash(&mut self, key: String) -> &String {
         self.map.get(&key).unwrap()
     }
 }
 ```

## Contract Mode

Now write contract have two mode:

- **normal mode**

Contract with normal mode limited data storage format, and user can write contract more
convenient. User can use all macros except `advance_contract`.

- **advance mode**

Contract with advance mode not limited data storage format, and it must open advance
feature with `fvm-std` and 'fvm-macros' lib to use this mode. The execute speed of contract
in this mode would be fast then the contract in normal mode.