Rank is wrong for Rational matrices

[Jean_Michel]

@andreasnoack

As people have already pointed out, the current rank function is for computing the floating point rank and when it can be hard to combine convenient promotion while correctly limiting the input, e.g. we can’t just require the elements to be <:AbstractFloat since that would exclude complex numbers and quaternions based on floating point numbers.

This is an important problem for me. To make the julia library friendly to other mathematics than floating-point numbers, it is needed to be able to dispatch on “approximate numbers” (Floats, Complex of Floats, Quaternions of Floats) versus “something else” (Rationals, user-defined types like algebraic numbers, rational fractions, whatever). The grey zone will be Integers, which are often treated as Floats, but not always, by Julia (I think BigInts should never be treated as floats). Has there been any thoughts along these lines among developers/designers of the language?