Make n_unkown usable

[tthsqe12]

• Optimize in libsingular_julia_jll conversion between n_algExt and nf_elem/fq_nmod
• Identify where singular can crash from n_unknown coefficients and have it raise an error instead of crashing.
• Discuss the possibility of having the missing functionality of n_unknown added to the coeffs struct via function pointers.
• New singular_jll including the above change(s)

[thofma]

Here is an example where using n_unkown creates a nice segmentation fault without stack trace for the innocent user.

julia> F, = Singular.Nemo.FiniteField(2^17 - 1, 2, "a");

julia> Fx, = PolynomialRing(CoefficientRing(F), ["x", "y"]);

julia> M = Matrix(Fx, [-1 0; 0 -1])
[131070, 0
0, 131070]

julia> Singular.LibFinvar.invariant_ring(M, [0, 0, 0])

signal (11): segmentation fault
in expression starting at REPL[33]:1
unknown function (ip: (nil))
Allocations: 21701572 (Pool: 21696118; Big: 5454); GC: 22
[1]    4490 segmentation fault (core dumped)  julia

I got this while working on the invariant rings in Oscar.

[thofma]

Note that this is crucial if we want to make Singular useful for Oscar. The situation with the segmentation faults is very off-putting for users.

[tthsqe12]

Here is a much simpler example, also calling a null pointer.

julia> r, _ = PolynomialRing(Singular.Nemo.QQ, ["x", "y"])
(Singular Polynomial Ring (Coeffs(17)),(x,y),(dp(2),C), spoly{Singular.n_FieldElem{Nemo.fmpq}}[x, y])

julia> Singular.LibRing.addvarsTo(r, r, Any["a", "b"], 0)
low_level_caller_rng: called
low_level_caller_rng: preparing arguments
low_level_caller_rng: calling singular

signal (11): Segmentation fault
in expression starting at REPL[3]:1
unknown function (ip: (nil))
Allocations: 12711430 (Pool: 12708583; Big: 2847); GC: 13
Segmentation fault (core dumped)

seems to work fine with native coeffs;

julia> r, _ = PolynomialRing(QQ, ["x", "y"])
(Singular Polynomial Ring (QQ),(x,y),(dp(2),C), spoly{n_Q}[x, y])

julia> Singular.LibRing.addvarsTo(r, r, Any["a", "b"], 0)
low_level_caller_rng: called
low_level_caller_rng: preparing arguments
low_level_caller_rng: calling singular
low_level_caller_rng: returned from singular
2-element Vector{Any}:
 Singular Polynomial Ring (QQ),(x,y,a,b),(C,dp(4))
 Dict{Any, Any}()