making HAL faster

[nhejazi]

clever things that might help us be fast:

• lassi_fit_cd should accept an index and perform fitting on that subset
• make things into a core class as per #20
• benchmark further cases of n, p for how lassi is performing vs glmnet
• coordinate descent wrt cv-risk -- only do things that minimize cv-risk
• use of R code for cv_lasso might be (probably is) causing significant delays
• could consider moving CV steps into RcppParallel instead of origami
• Wainwright http://jmlr.csail.mit.edu/papers/volume17/15-605/15-605.pdf

[nhejazi]

Stale -- closing since we've deprecated lassi as of v0.4.0+