Moved pull implementation from stream.hpp to new file pull.hpp

[codeinred]

This is a more major refactoring of stream.hpp, which is why I'm doing it on it's own branch, and initiating a pull request. This branch splits the implementation of stream and pull into separate files. It also removes the function pull.toStream(), which just invoked stream's constructor. This created an unnecessary cyclic dependency between the two classes. All instances of pull.toStream() were replaced with a direct call to streams constructor.

Questions:

• What is bot (declared as an empty union), and how does it differ from top (declared as using top = std::monostate)?
• Can we remove eff.hpp? It doesn't seem to do anything. It contained the definitions for bot and top, but I moved those into stream.hpp and pull.hpp respectively.

[s5bug]

top = top type, meaning it has only one inhabitant. An example of top is a struct with no fields. The only inhabitant is the single empty struct.

bot = bottom type, meaning it has zero inhabitants. An example of bot is a union with no entries. There's no way to initialize this union. (So, a task<bot> is a task that never completes. A vector<bot> can only ever be an empty vector. Etc.)

[s5bug]

Regardless, pull should not be exposed as a primary type. I will explain why in tidy/stream.docs.