Rename JsonStreamContext as TokenStreamContext

[cowtowncoder]

(and probably similarly sub-classes, into TokenWriteContext / TokenReadContext)

Continuing with theme of reducing "json" in names where not actually relevant, let's rename low-level context object. Part of the reason this renaming is important is the addition of two new abstractions:

• ObjectWriteContext: context databind passes for serialization state, configuration
• ObjectReadContext: context databind passes for deserialization state, configuration

which are more compatible with "token stream" naming, over generic stream and read, write.