String escaping

[webmaster128]

As documented in https://github.com/japaric/serde-json-core/blob/238c8fe/src/lib.rs#L46, strings are not escaped in serialization. This leads to broken JSON results when e.g. a " character exists as part of the string content.

If someone wants to tackle this, feel free to copy the implementation from CosmWasm#10.

You probably need to work on unescaping at the same time (which is hard and not zero-copy) if you want deserialize(serialize(original)) == original. For a seralize-only use case however, escaping is an important independent step.