Move the assignment parser into a separate module.

Move the assignment parser into a separate module.
When assigning structures, copy them using the primary register when the
size is 1, 2, or 4.
When assigning structures, allow structures by value when the size is 1,
2, or 4. These structures are expected to be in the primary register. The
only case when this can happen is by return from function, so this change
makes div() work.

git-svn-id: svn://svn.cc65.org/cc65/trunk@1477 b7a2c559-68d2-44c3-8de9-860c34a00d81