Redesign operation pipeline

[matt-ramotar]

Closes #6

Changes:
Introducing OperationPipeline and MutableOperationPipeline. Also introducing Actions to update the internal pipeline through Dispatcher.

Significance:
By encapsulating mutable operations within the internal paging system, integrating the pipeline more tightly with Pager, and streamlining modifications to the pipeline through the Dispatcher:

• The public API is more straightforward and easy to use (see Simplify core API #5)
• The consumer can update operations more safely