[position] Move make_move to its own module: position::make_move

Rework sight.rs and add a new module, movement.rs, to calculate piece sight and
movement. I discovered during this process that "sight" and "movement" are different
because pawns (in particular) can move in ways that don't follow their sight lines.
The routines in the movement module account for this, but also pass through to the
sight routines for other pieces.

position/src/lib.rs

@@ -3,6 +3,7 @@
 mod check;
 mod display;
 mod move_generator;
+mod movement;
 mod position;
 mod sight;