Replace "hofs" and "fofs" terms with better names

[eriknw]

I don't like the names "hofs" and "fofs". These terms should not be presented to the user from the API or documentation. We need to come up with better names, and a few ideas are listed below:

• functionals, functions
• functors, funcs
• operators, ???
• high*, low*
• chain*, ???
• ???, leaf

Note that these terms may be part of a function name, such as addhofs, addfofs, which may be changed to addfunctionals and addfunctions. Issue #2 also introduces functions that would convert between fofs and hofs, and coherent language would be great.

[eriknw]

I'm going with metafuncs and funcs, but am willing to change them if better names are found.