sci-libs/m4ri: new package for dense matrix computations over F2.

sci-libs/m4ri: new package for dense matrix computations over F2.

This package was imported from the sage-on-gentoo overlay with only a
few changes:

  * Updated from EAPI=6 to EAPI=7, and subsequently moved
    virtual/pkgconfig to BDEPEND.
  * Put the --enable-png flag behind USE=png.
  * Enabled the test suite, which passes out-of-the-box for me.
  * Update the HOMEPAGE to point to bitbucket; it was redirecting.
  * Updated LICENSE to GPL-2+ from GPL-2 (based on file headers).

The SageMath package can detect and use the system copy of m4ri, which
means that this package should eliminate pointless rebuilds of m4ri
for users of SageMath on Gentoo.

Closes: https://bugs.gentoo.org/704500
Package-Manager: Portage-2.3.79, Repoman-2.3.16
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>