LinearAlgebra support for cross product of 3-tuples

[MilesCranmer]

The current behavior is:

julia> using LinearAlgebra

julia> norm((1, 2, 3))
3.7416573867739413

julia> cross((1, 2, 3), (4, 5, 6))
ERROR: MethodError: no method matching cross(::Tuple{Int64, Int64, Int64}, ::Tuple{Int64, Int64, Int64})
The function `cross` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  cross(::AbstractVector, ::AbstractVector)

with this PR:

julia> cross((1, 2, 3), (4, 5, 6))
(3, -6, 3)

The PR is pretty minimal, basically all I did was move the current code to the 3-tuple version, and then wrap it with a collect in the vector version.

[LilithHafner]

We should definitely accept tuples if the length is known at design time (which it is). This should have tests. Tagging triage because it's a new API

[MilesCranmer]

I added the test to the jldoctest. Lmk if that's good enough or not. The code for the vector unittests will now hit the tuple version though so should be tested elsewhere anyways.

[LilithHafner]

Sounds good enough to me.

[fredrikekre]

These types of PRs are usually rejected with a pointer to StaticArrays.jl.