Allow CRT_list() to be called with one argument

[user202729]

As in the title.

Opinion: should we allow crt() / CRT() to be called with one list also? Note that currently it is callable with two lists and does the exact same thing as CRT_list().

Opinion: is returning Mod(0, 1) for empty list a sane idea? Problem is later we may want to implement (pseudocode)

CRT_list([mod(x-1, x^2+1), mod(x+1, x^2-1)])

and in that case the correct thing to return is not clear when the list is empty.

More generally, if it's given elements x₁ ∈ R/I₁, …, xₙ ∈ R/Iₙ where Iᵢ = xᵢ.parent().defining_ideal() and all R = xᵢ.parent() are the same for all i, it should returns x ∈ R/(I₁.intersection(I₂).intersection(I₃).….intersection(Iₙ)). But this is not well-defined when values is empty.

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[tscrim]

A few small things.