`(_)arb_mat_entrywise_(not_)is_zero` doesn't make sense

[albinahlback]

Why is the output an fmpz_mat? Moreover, the documentation and header implies no leading underscore, but the source file for arb_mat_entrywise_is_zero has an underscore in it.

I suggest we either remove these functions or set the output to something such as truth_mat_t (if this would exist).