Allow key deserialization from owned strings

[sgeisler]

This patch allows the underlying parser impl to either return a String or a &'de str. Deserializing json encoded keys from Readers fails otherwise.

[sgeisler]

Adding no-std support made the PR quite ugly. Does anyone have an idea how to implement it more elegantly?

[elichai]

Damn It.
So I spent a few hours re-implementing this such that it will work without the no-std trick (and will also work for Owned bytes(ie Vec))
and after I finished apparently PublicKey already have a working impl of its own outside of the macro which is almost exactly the same as the one I wrote: https://github.com/rust-bitcoin/rust-secp256k1/blob/master/src/key.rs#L392

I think I'll remove the macro all together because only SecretKey uses it and the other types have their own shticks.
And I'll add more tests that actually test this bug

[sgeisler]

Oh, damn it, I didn't notice the macro was only used there, that's ugly indeed. Do you want to take this over? If not I can do it too.

[elichai]

Oh, damn it, I didn't notice the macro was only used there, that's ugly indeed. Do you want to take this over? If not I can do it too.

Please see #218 :)

[sgeisler]

I think #218 is the better version now.