faster implementation of rank(::QRPivoted) fixes #1128 (#1129)

(fixes #1128)
The current implementation of `rank(::QRPivoted)` is slow and inefficient because of repeated calls to `A.R`, each of which triggers memory allocation. This PR fixes that by using `A.factors` instead, which does not cause memory allocations. The proposed implementation is orders of magnitude faster and avoids unnecessary memory allocations altogether.

Author: ajinkya-k

src/qr.jl

@@ -535,11 +535,11 @@ function ldiv!(A::QRCompactWY{T}, B::AbstractMatrix{T}) where {T}
     return B
 end
 
-function rank(A::QRPivoted; atol::Real=0, rtol::Real=min(size(A)...) * eps(real(float(one(eltype(A.Q))))) * iszero(atol))
+function rank(A::QRPivoted; atol::Real=0, rtol::Real=min(size(A)...) * eps(real(float(eltype(A)))) * iszero(atol))
     m = min(size(A)...)
     m == 0 && return 0
-    tol = max(atol, rtol*abs(A.R[1,1]))
-    return something(findfirst(i -> abs(A.R[i,i]) <= tol, 1:m), m+1) - 1
+    tol = max(atol, rtol*abs(A.factors[1,1]))
+    return something(findfirst(i -> abs(A.factors[i,i]) <= tol, 1:m), m+1) - 1
 end
 
 # Julia implementation similar to xgelsy