Redundant structs

[vimmerru]

For the current moment all API functions implemented as static structs methods in dedicated submodule. It causes unnecessary usage of structs in code. We can just have public functions in submodule:

indy::wallet::Wallet::create("{}", "{}").unwrap();

can be just replaced with

indy::wallet::create("{}", "{}").unwrap();

It will look much more nature to Rust.