fiber: refactor fiber and builder

Builder changes:
- Builder::callee -> Builder::func / Builder::proc: two different
  methods depending on if function returns a value or not
- removed Builder::start_unit, Builder::defer_unit

Fiber changes:
- fiber::defer_unit -> fiber::defer_proc
- replace Immediate, Deferred, ImmediateUnit, DeferredUnit with a single
  parametrized Fyber<C, I> struct

Other changes:
- fiber::start_unit -> fiber::start_proc

Author: gmoshkin