WIP: Refactors QR code

Deprecates `qrp`, `qrpfact` and `qrpfact`!
Adds trailing `pivoted::Bool` option to `qr`, `qrfact` and `qrpfact!`

Reorganizes QR factorizations section so that methods common to `QR`,
`QRPackedQ`, `QRPivoted`, `QRPivotedQ` are grouped together.

TODO: also work on pivoted Cholesky `cholpfact`, etc.