add dagcbor::validate() function.

[raulk]

This utility function validates that a supplied input is syntatically valid DAG-CBOR, while attempting to deserialize as little as possible.

• It has to deserialize numbers to validate they're minimally encoded.
• It validates that map keys are strings, and map values are valid DAG-CBOR.
• It validates that list elements are valid DAG-CBOR.
• It does not validate UTF-8 encoding of strings.

Closes #158.

We may want to remove the recursion here to improve security against untrusted inputs.

[vmx]

I haven't really looked at the code yet, just a general note: for FVM we don't use the DAG-CBOR implementation from this library, but we use https://github.com/ipld/serde_ipld_dagcbor. So if validation functionality would be added, it should be added there instead.