Support for json.parse with extensive round-trip tests

[TristonianJones]

Support for json.parse with extensive roundtrip tests.

If the value can be parsed as JSON, it returns an optional_type with a value.
There are two flavors of json.parse, one which returns a dyn value, and one
which takes a type identifier and attempts to parse the JSON value into the type.

The type-specified version of json.parse supports both native and protobuf type
definitions.

Benchmarks included under ext/design/encoders.md