Add NumberColumns and NumberRows to MatrixObj interface

[fingolfin]

These are meant to be used instead of Length and RowLength.
We retain RowLength for now, to keep existing code (and packages!)
working, but the plan is to remove this eventually.

Also provide default implementations of both for IsMatrix.

[ssiccha]

Looks good to me!

[codecov]

Codecov Report

Merging #1657 into master will increase coverage by 0.17%.
The diff coverage is 17.64%.

@@            Coverage Diff             @@
##           master    #1657      +/-   ##
==========================================
+ Coverage   64.16%   64.33%   +0.17%     
==========================================
  Files        1001     1001              
  Lines      325891   326672     +781     
  Branches    13196    13211      +15     
==========================================
+ Hits       209098   210174    +1076     
- Misses     113442   113646     +204     
+ Partials     3351     2852     -499

Impacted Files	Coverage Δ
lib/vec8bit.gi	48.71% <0%> (-0.43%)	⬇️
lib/matobjplist.gi	0% <0%> (ø)	⬆️
hpcgap/lib/vec8bit.gi	47.86% <0%> (-0.4%)	⬇️
lib/vecmat.gi	67.18% <0%> (-0.24%)	⬇️
hpcgap/lib/vecmat.gi	64.67% <0%> (-0.21%)	⬇️
lib/matobj.gi	9.03% <37.5%> (+0.59%)	⬆️
src/saveload.c	56.1% <0%> (-1.61%)	⬇️
src/iostream.c	45.34% <0%> (-1.3%)	⬇️
src/system.c	53.1% <0%> (-1.23%)	⬇️
src/vector.c	92.16% <0%> (-0.4%)	⬇️
... and 65 more