Improved StepExecutions, available outside test scope

[jglick]

I found myself using the StepExecutions utility a lot in jenkinsci/workflow-cps-plugin#834 but there were some limitations that made it awkward:

• You needed to return null; even for synchronous steps which were not designed to return a value (the most common case), forcing use of a block-form lambda even for bodies consisting of a single statement.
• The requirement that the body lambda be Serializable made for some awkward idioms. This was never productive since the body is used only in StepExecution.start, so even if the execution gets serialized, there is never a reason to deserialize the body.

After making those improvements I realized that the utility could actually be useful for production code, so opening it up outside of test scope. (Not helpful for existing steps since these need to retain serial form compatibility of the StepExecution implementation, except perhaps in the case of SynchronousStepExecutions.)