Faster regs

[laszlosandor]

Simply the use _regress for both reg and areg, speeds things up, esp. the latter (often 3x).

I am not aware of use cases where this breaks anything. E.g the -areg- wrapper does an extra round of _regress without the FEs to report an F-test on them, but no preparation or reporting is broken otherwise.

_predict is also there for predict, though the main speedup is from -_regress, absorb()-

[michaelstepner]

Cool! I'll get to these things after generals.