Move 'apply_if' and 'pass_if' functions into 'maybe'.

[nmbooker]

The 'maybe' module will contain anything to do with:

• lifting and chaining existing functions so they can handle possible failure (maybe there's a value). Functions are:
  • maybe
  • maybe_c
  • odoo_maybe
  • odoo_maybe_c
• conditionally skipping function application according to your own criteria (maybe I'll apply the function). Functions are currently in the 'passthrough' module:
  • pass_if
  • apply_if

I may want to rename the latter two functions when I move them into maybe. I'm thinking 'apply_unless' would be better than 'pass_if'.