Moving to spec 1.0.0 requires more complicated conversions, so
put the converter determination logic in a separate module that
any of the types can call to convert between arbitrary versions.
This is necessary to ensure the types don't have import cycles.

This also implements downconversion, which we never claimed
*not* to support, but didn't implement.

It also verifies that the Result object unmarshalled by
each result type's NewResult() function is actually supported
by the result type. This is a potentially breaking change as
this was not previously done, and for example may now fail
attempts to read a cached PrevResult written by a previous
version.

Signed-off-by: Dan Williams <dcbw@redhat.com>

Only change this time is removal of the IPConfig "Version" field
which was deemed redundant.

Signed-off-by: Dan Williams <dcbw@redhat.com>

The testcase checking if a prevResult was invalid is not actually
useful, because it was testing that an empty prevResult could be
unmarshalled. But due to an oversight, prevResults may not have
a CNIVersion key, which is all we can check for validity.

Signed-off-by: Dan Williams <dcbw@redhat.com>

…esses

Redundant and easily determined from the address itself.

Signed-off-by: Dan Williams <dcbw@redhat.com>