feat: make #[hanlde_result] work with fully qualified paths

[itegulov]

Fixes #852

Opening this as a draft for now since there is one unresolved thing with FunctionError impl for anyhow::Error. There is no AsRef<str> implementation for anyhow::Error and we can't implement it ourselves (or FunctionError itself) due to conflicting implementations (life is hard without specialization...).

[uint]

AsRef<str> can't be implemented on anyhow::Error anyway. Since an anyhow::Error might have to build a string dynamically, there's no way to guarantee a cheap pointer cast there.

@itegulov @frol What's the rationale for having impl AsRef<str> as the SDK error type in the first place? Is it to encourage devs to produce unhappy paths that are cheap? Or is it just convenience on our side for easily getting something that we can call near_sdk::env::panic_str on? I'd like to understand our constraints and needs here.

[uint]

Stale PR. Closing.