Disallow dup col names in Frames

Add support functions to auto-rename cols
Exec2 cbind auto-renames cols to avoid dup names
Remove dead import
FVec to VA converter

First cut, expect bugs.  Hooks H2OHexKey RequestArg which now accepts
Frame keys and returns a VA.  No effort to be smart on compression, or
noticing auto-converts FV->VA->FV.
Merge branch 'master' of github.com:0xdata/h2o
Force column renames to keep The World Safe