Add type signatures to GoodData.REST.Session.
Move GoodData.REST.Session to GoodData.Session.

GoodData.REST shall be reserved for actual REST API names.
GoodData.REST.Types.Basic: Add more instances to BOOLEAN; add INT.
Add typed URI (with optional content) and a generic expander.
Util.JSON: Replace tagged with a nicer composable tag.
Util.JSON: Add untag and optionalPropBy.
GoodData.REST.Types.MD.Obj: Add Meta, AttributeDisplayForm, Attribute, Key, Column, etc.