QR factorization enhancements

[fredrik-johansson]

Things to follow up #2401:

• Add the Householder algorithm, which is generally considered superior to Gram-Schmidt, especially for ill-conditioned matrices. I'm not sure if there is a big difference in arbitrary-precision arithmetic, but it's worth having both algorithms to compare.

• Handle complex numbers the expected way, i.e. generating unitary matrices (requires adding _gr_vec_dot_conj and some other helper functions to do cleanly).

• Currently only full rank matrices are allowed. Either generalize the current implementation, or add separate rank-revealing functions.

• Posteriori enclosures for ball arithmetic.

• Add a square root-avoiding algorithm (keeping norms factored out) for exact/symbolic fields.