Initial support for binding maps.
Context-sensitive map binding.
SimpleBuildStep needs special handling.
Throwing out Jenkins.getDescriptorList for now and doing custom filtering on Descriptor extensions.
Do not misinterpret a struct map passed as the value of a single mandatory parameter as a map of parameters.
Cannot omit the default parameter name for CoreStep & GenericSCMStep.
Or if you do, the value must not syntactically be a map expression (as opposed to a variable referring to a map).
This is due to an apparent limitation in Groovy syntax.
(Manifested as an UnsupportedOperationException with opaque stack trace in groovy-cps.)