Fix memleaks and brokenness of non-FLINT methods after #16803

[jdemeyer]

non-FLINT methods are seriously broken after #16803:

sage: a = zero_matrix(QQ,3)
sage: b = identity_matrix(QQ,3)
sage: a.minpoly()
x
sage: b.minpoly()
x - 1
sage: a.minpoly('y')
y - 1

Also, using linbox or iml and modifying a matrix leaks memory:

sage: v = vector([0]*1000)
sage: M = identity_matrix(ZZ,1000)
sage: while True:           
....:    _ = M._solve_right_nonsingular_square(v)
....:    M[0,0] = 1
....:    print get_memory_usage()

Also, use type int for _hnf_mod() (anything larger than 2^31-1 isn't supported anyway).

[jdemeyer]

Last 10 new commits:

​9b31003	Check 'implementation' argument
​a2860d8	Undo changes to padics from #16803
​342d152	Remove unneeded imports
​95b431d	Put back old multiplication code (as requested by William Stein)
​5b77c78	Restore doctest
​7843e3f	Fix fmpz_mat_t declaration
​192c8e6	Missing comma between command-line arguments
​9604069	Merge commit '192c8e6f5c6e5f0bd0dd02d51b5f103924a408ef' into ticket/17090
​c46a4d5	Some reformatting
​a8c707d	Fix initialization of matrices and fix _hnf_mod()

[jdemeyer]

Found an easy example showing that there is really a problem in practice.