Merge branch 'u/SimonKing/meataxe' of trac.sagemath.org:sage into t/1…

…2103/meataxe

* 'u/SimonKing/meataxe' of trac.sagemath.org:sage:
  Fix pickling of meataxe matrices
  Remove overcautious commands in spkg-install; rely on default error return values in matrix_gfpn_dense
  Use and propagate specific return values on error in matrix-related MeatAxe functions.
  Fix doctests when meataxe is installed
  Fix computation of row-reduced echelon form
  Doctests and error handling for MeatAxe
  Improve echelon computation in MeatAxe, and fix some compiler warnings
  A full wrapper for MeatAxe matrices
  A very basic MeatAxe Cython wrapper
  Implement and use Strassen-Winograd matrix multiplication in MeatAxe
  An optional MeatAxe package