Imports and Nits

[drspacemn]

Moving PR comments to this issue.

Suggest CamelCase interface names as follows cairo-lang.org and OZ naming conventions.

Remove unused imports.
And import OZ utils from openzeppelin.token.erc20.library import ERC20

Can remove internal tempvar binding with return in the if block.

Nit: suggest removing _storage from variables annotated @storage_variable