Changes to &info.

Added explicit variable for positive INFO arguments, which can now be
checked by the return form.

When CONDITION is NIL, positive INFO arguments are ignored.

This was necessary because some LAPACK return meaningful results even
for a positive INFO, which is then something like a warning (eg an LU decomposition of a singular matrix).
Made LU proceed with singular matrices.  Added a test case.
Exported LU-U and LU-L.
Rewrite of pivoting.

- renamed slot to ipiv-internal to indicate that it's LAPACK's representation
- ipiv-internal is not exported
- exported function now uses indexes from 0, may be used with CL-SLICE:SLICE.
Added test, fixed IPIV code.

Apparently LAPACK follows a different convention compared to what I was thinking of.
Added IPIV-INVERSE, examples, tests.