Improve multiplication for ZZMatrix

[JohnAAbbott]

Below is a "ridiculous" example: compute the square of a 1000x1000 identity matrix with the top right hand entry set to a large integer. The computation immediately grabs a lot of memory (about 40Gbyte on my computer), runs for a while, then is killed (presumably by systemd-oomd). The computation should be faster, and should not require so much RAM space.
Here is the OSCAR input to reproduce the problem -- it completes with sz = 500; or with exp = 5000

sz = 1000;
exp = 10000;
M = identity_matrix(ZZ,sz);
M[1,sz] = ZZ(10)^exp;
@time Msq = M^2;

This is much simplified from the original problematic computation -- a product of two upper triangular matrices with relatively few quite large entries.

[fredrik-johansson]

This is a FLINT issue. fmpz_mat_mul chooses an algorithm based on the dimension and maximum height, but does not currently account for the distribution of heights.