API improvements

[anweiss]

Addresses #105 and #114

[anweiss]

@itamarst FYSA, this PR introduces some API improvements per #105 so you can now do something like

let cddl_schema = CDDL::from_slice(
  r#"
    foo = {
      bar: tstr
    }
  "#.as_bytes(),
)
.unwrap();

let documents = [r#"{ "bar": "foo" }"#, r#"{ "bar": "foo2" }"#];

documents
  .iter()
  .all(|doc| cddl_schema.validate(doc.as_bytes(), None).is_ok());

Let me know what you think

[itamarst]

I can probably create a branch off of your branch, will try that.

[itamarst]

That is, create a branch of pycddl that uses branch of cddl, to try the new API.

[itamarst]

I tested CDDL::from_slice with my current approach of parsing the schema for every validation. It works, simplified the code slightly.

[itamarst]

(The lifetime requirements is still making interop with PyO3 hard, though, since it's explicitly not supported: PyO3/pyo3#502. So #104 is still blocker on keeping pre-parsed CDDL around.)

[itamarst]

For reference, here's the branch using this branch: https://gitlab.com/tahoe-lafs/pycddl/-/blob/12-persistent-schema-instead-of-parsing-schema-on-each-validation/src/lib.rs

[anweiss]

Thanks for testing! Yea, I'll be working through #104 once I get this PR finished up.