some tidying up in AST
more special cases of walking an R ast
reorg, dev notes, and slot assign impl
missing paren
Minor fix in finish frame method.

Enum column was interpreted as string column.
Add tests for model metrics.
Merge branch 'brandon-fixes'

* brandon-fixes:
  PUB-DEV 16 - Fix ParseTimeTest to be timezone independent.  Given that the local timezone is determined by each node, rather than push a desired timezone to the parser, this just fixes the test so that the hardcoded times are adjusted to reflect the local timezone (which the parser will impose on the times read from a file).  Of course, why anyone would live outside of PST is beyond me.
slot assign backend impl
utility functions for handling spans and series with single numbers
methods for removing and subRef'ing vecs
make public makeSimpleTransf and makeCon(double)
complete TODO by porting IAE to backend (rather than R side)
edit parameters to match dev
add ability to set character vals in slot assign
fix R units and switch some to NOPASS
dig out frame names for IDs from ASTs
some cleanup and finishing touches on slot assignment
HEX-1932: fix from h2o1 ported to h2o-dev
fix key passing in as.h2o
We need to override DataFrame#toString explicitly to avoid pulling
content of whole frame.
A simple wrappers for client mode.