Remove Coder.Context from the public API

Justification:

• Contexts add confusion and complexity to the public API (e.g. https://issues.apache.org/jira/browse/BEAM-1448)
• Leaf (user-written) coders are nearly always nested.
• Coders are being removed from sources, which was the initial need for context.
• It is unclear how much value, if any, this provides for internal performance.
• There may be performance (as well as simplification) gains in removing this for the Fn API.

Fully removing this distinction from the internals can be defered until the last bullet points are more completely investigated.