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.

Crates using this library can further add any implementations of
TryFrom<RpcValue> for their own types and  get a uniform way to
do all types conversions from RpcValue.