types: clean up JSON unmarshaling story for extension types

[patjakdev]

Issue #, if available: None

Description of changes:

Create a generic unmarshalExtensionValue() helper function that supports all three forms of extension value JSON encoding:

• Explicit: { "__extn": { "fn": "decimal", "arg": "1234.5678" } }
• Implicit object: { "fn": "decimal", "arg": "1234.5678" } }
• Implicit string: "1234.5678"

Use it to flesh out the implementations of UnmarshalJSON for IPAddr and Decimal. Note that the implicit decoding will never actually be invoked until we have schema-driven decoding for entities.

[apg]

nit: you could combine this into one var declaration

Suggested change

	var zeroT T
	var arg string
	var (
	zeroT T
	arg string
	)

[patjakdev]

I think I prefer the two line version...

[philhassey]

LGTM