BREAKING CHANGE: standardize the code with cargo clippy

[mateolafalce]

Hello!
I see in the #36 a proposal for standarizartion of the code with clippy. I found it a good idea so I think that to maintain standardization it would be convenient to add continuous integration to this new requirement into the build.yml file

[mateolafalce]

In addition, I fix:

error: redundant redefinition of a binding `method`
  --> src/contract/nft/item_contract.rs:48:13
   |
48 |             let method = method;
   |             ^^^^^^^^^^^^^^^^^^^^
   |
help: `method` is initially defined here
  --> src/contract/nft/item_contract.rs:42:13
   |
42 |         let method = NftItemContractMethods::GetNftData.into();
   |             ^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_locals

[SlavikBaranov]

Hey @mateolafalce I agree, we should definitely use clippy. I've already mentioned in #36 that we'll incorporate these changes in 0.13 and enable it in CI right away.