math/gfreduce.c: Refactor and document.

math/gfreduce.c: Refactor and document.

There was an unfortunate amount of code duplication in the old code,
particularly around the issuing of LSR instruction sequences.  Gather
the relevant state into a structure, and split the repeated code into
separate functions.

A casualty of this refactoring work is the work put into calculating the
`liv' slot in the `gfreduce' structure, carefully calculated to be a
pointer immediately after the first `STORE' instruction.  Alas, this
information is completely useless.

Also add a bunch of commentary explaining the underlying theory.