Special case for 2x3 matrix solve

[weymouth]

I'm doing some coordinate transformation stuff that requires the solution of a 2 by 3 system. Right now, all rectangular SMatrix systems are done by LinearAlgebra and the allocations mean I can't run this on the GPU. I coded this version which seems to be working and non-allocating.

@inline function (\)(a::SMatrix{2,3}, b::SVector{2})
    # columns of a'
    a1,a2 = a[1,:],a[2,:]
    
    # Q,R decomposition
    r11 = norm(a1)
    q1 = a1/r11
    r12 = q1'*a2
    p = a2-r12*q1
    r22 = norm(p) < eps(r11) ? one(r11) : norm(p)
    q2 = p/r22

    # forward substitution to solve R'v = b
    v1 = b[1]/r11
    v2 = (b[2]-r12*v1)/r22

    # return solution x = Qv 
    return q1*v1+q2*v2
end

I'm happy to add this to solve.jl and do a PR if it would be helpful.