Remove feature gate for Encoding

[desaikd]

Encoding is gated under a feature "experimental-lazy-reader". This needs to be removed for 1.0 release.
Encoding and LazyEncoder share some similar functionalities and needs to be modified to keep only one of these.

[zslayton]

The practical effect of this change will be that Element::read_* and Element::write_* methods will be able to accept a ReadConfig and WriteConfig respectively, allowing users to (e.g.) specify a Catalog or request a particular version of Ion. These types also future-proof these methods by giving us an avenue by which to add more options without modifying the API.

[popematt]

If we remove the feature gate for Encoding, then we need to add a feature gate specifically to the Ion 1.1 encodings (until the Ion 1.1 spec is finalized). Currently, the Cargo.toml file does not list any feature for Ion 1.1.

[zslayton]

Interestingly, I've learned that it's possible to expose a type that's generic over E: Encoding without making Encoding itself pub, which is nice. This is currently the case with WriteConfig<E: Encoding>. In light of that, I'm closing this for now. If anyone disagrees, please re-open this and we'll discuss.