add recall list manager

This commit adds a dedicated recall list manager class.

The purpose of this is to

- Simplify the code that interacts with the recall list. Prior to
the commit most such code was based on iteration with explicit
iterators, and called global helper functions implemented in
unit.cpp to wrap the code that finds a unit in a vector. It turns
out that interacting with the recall list was the *only* use of
that code, so we make it a member function of the recall list
manager and take it out of unit.cpp.

Most of the code that touches the recall list was previously
7 or 8 lines with a for loop, now it tends to be 1 or 2 lines,
although further refactor may be possible.

- Improve encapsulation. This makes it possible to track how
other classes are interacting with the recall list, and may
make it easier to debug recall list problems by adding debugging
output to the class.