Implement serde Serializer/Deserializer::is_human_readable()

[desaikd]

Related PR #791

Description of changes:
This PR works on adding is_human_readable() for Serializer and Deserializer in serde implementation of Ion.

List of changes:

• Added is_human_readable property in ValueSerializer based on WriteConfig.
• Added is_human_readable property in ValueDeserializer based on SystemReader's next_item().

Test:
Added a unit test that converts to/from binary and text as well as compares for roundtrip.

Note for reviewer:
Miri test here is failing due to writer code in lazy writer and is unrelated to current change.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[zslayton]

There are a couple of clippy warnings to address, but otherwise this LGTM! At a glance I don't think the Miri warnings are caused by this change--I'll look into those separately. They need to be addressed before we cut another release.