DSP API overlapping arrays + inv() and neg()

Hey Marcus,

We were just discussing our implementations of the DSP API on IRC with
Jens. We were thinking about the problem of source and destination arrays
being the same or overlapping, and came to the conclusion that it's not a
good idea to allow this because it will be impossible to define this
behavior without restricting implementations too much (we shouldn't require
the implementations to guarantee that things are run in 0 - n order,
otherwise we'll exclude parallelization). However, avoiding creating
unnecessary copies of data is a good thing, so we thought that a suitable
solution would be that dropping the destination parameter(s) would imply
that destination and first source are the same array.

That, however, presents another problem, because sub and div give different
results if you change the order of the sources (obviously). For this,
inv(erse) and neg(ate) would be useful additions to the API.

Cheers,
Jussi