Implement TryFrom on RpcValue

Implements TryFrom<&RpcValue> and TryFrom<RpcValue> for various
basic types and their references.

TryFrom<&RpcValue> would be useful e.g. for RPC requests params
parsing, where `RpcMessge::param()` returns `Option<&RpcValue>`.

TryFrom<RpcValue> can provide more efficient implementation for
larger datatypes (Blob, List, Map, String), where the returned
value is just moved out of the underlying Box without cloning.