Binding codegen: RFC6020 defines the order of evaluation for union members (binding spec v2)

+++ This bug was initially created as a clone of MDSAL-49 +++

Our codegen requires users to manually instantiate the builder for union types, which parses a string. RFC6020 (http://tools.ietf.org/html/rfc6020#section-9.12), specifies the following:

When a string representing a union data type is validated, the string
is validated against each member type, in the order they are
specified in the "type" statement, until a match is found.

This removes enough ambiguity, such that we can auto-generate a proper builder. Generate the builder (still in src/main, as we need to maintain compatibility).