Implement deserialize_any

[liarokapisv]

This PR adds support for deserialize_any which is useful when trying to work around the lack of untagged enum support.

Mostly follows serde-json 's approach without the inlining.

There is definitely room for improvement when it comes to the implementation:

• We can inline the delegated methods and avoid redundant checks
• We can probably avoid the multiple delegations to from_str implementations when parsing for numbers.

This will probably require some cleanup, it currently uses a Visitor adaptor to take advantage of the existing deserialize_fromstr macro.

[eldruin]

Sounds interesting to me, thank you.
Could you add tests for the other types like string, map, etc?