Add gr_vec_set_str and gr_mat_set_str

[fredrik-johansson]

Allow parsing vectors and matrices from strings in the same format we print them ([a, b, c] for vectors, [[a, b], [c, d]] for matrices).

>>> Vec(ZZx)("[x+1, (x+1)^5]")
[x+1, x^5+5*x^4+10*x^3+10*x^2+5*x+1]

>>> Mat(RR)("[[1/3, 0], [[0 +/- 1], pi +/- 1/10^4]]")
[[[0.3333333333333333 +/- 7.04e-17], 0],
[[+/- 1.01], [3.142 +/- 5.08e-4]]]

>>> Mat(Qp_padic_radix(7))("[[1, 2], [3 + O(7^10), 4/5 - O(7^8)]]")
[[1, 2],
[3 + O(7^10), 1152961 + O(7^8)]]

>>> Vec(Mat(QQ, 2, 2))("[[[1,0],[0,1]], [[0,1],[-1,0]]]")
[[[1, 0],
[0, 1]], [[0, 1],
[-1, 0]]]

Straightforward, mostly AI-generated patch.

Note that gr_set_str is overloaded to call these methods for vector and matrix rings, but the generic gr_set_str doesn't yet handle vector or matrix subexpressions for generic rings. So it's still not possible to parse polynomials with matrix coefficients, for example.