Cleanup Vec type system

Single _type field instead of a bunch of unrelated booleans.
Cleanup handling of mismatched Chunks within a single column.  This may
change the behavior of mixing datatypes within a column slightly (in all
cases this only matters when the datatypes are already clearly
confused).
Use makeZero() rather than makeZeros(1,...)[0]
Merge branch 'master' of github.com:0xdata/h2o-dev