Add support for 128bit numbers

[KodrAus]

Hi there!

I ran into a case recently where I needed 128bit number support in erased-serde so have taken a stab at implementing it here.

I've gone with a conservative route of defaulting the conditional trait methods even though I think the de::Visitor is actually private. This should be a non-breaking change.

Please let me know if you'd like me to throw a test case in there somewhere or if there's anything you'd like done differently.

[dtolnay]

Thanks!

None of the three traits Serializer, Deserializer, Visitor are possible to implement outside of this crate so you can remove the default impls of the new methods.

[KodrAus]

Done 👍 It also turns out I'd missed a few of the impl blocks the first time around

[dtolnay]

Nicely done. I published 0.3.6 containing this change.