MeatAxe: implement and then use in-place matrix operations

[fingolfin]

Consider code like this in lib/meataut.gi:

        a:=a - a[r,c] * nilbase[i] / nilbase[i][r,c];

where a and nilbase[i] are square matrices.

This is highly inefficient: first the scalar division / creates a new matrix; then the scalar multiplication creates a second new matrix; then the subtraction a - ... creates a third, which is assigned to a.

Instead this could be done with zero allocations (well, assuming the field operations don't require allocations, which is the case for finite field elements). For larger examples this makes a substantial difference.