Added ReadOnlyDesignTest
Cleanup

Replaced (almost) all occurrences of strings with numeric literals
when setting limits or a fixed value.

Strings were originally used in an attempt to boost confidence in the
results of the tests, because that seemed to make the tests resemble
manual XML configuration more closely. But if there were truly a
difference, then why even support the Object arguments instead of
limiting arguments to Strings?
Cleanup

Replaced occurrences of ParamCfg.builder().build() with
ParamCfg.getDefault().
SetFixedTest: add more tests

Showing that setFixed(null) unfixes the parameter.
SetFixedTest: add link to relevant test.
ParamCfg: add SChoice support
SetFixedTest: fix typo
ParamCfg: minor refactoring
Helper and MappingCfg: fix argument order

Supporting varargs requires the type to be the first argument.
Updated methods to be more consistent about ordering.