Cleanup fvec compression

Major comment upgrade.  Allocate side arrays in each Chunk.
Drop redundant min/max/hasFloat fields.
Drop redundant invalid/setInvalid calls.
Minor bugfix with setting NA into existing C0DChunk.
More cleanup of NewChunk

Allow Enums to compress any style; remove one 64K cap (may be more
limits).  Correct/cleaup _naCnt.  Tighter asserts, more comments.
Remove leaking Key from Bit-test
Uniformly cleanup enums & numbers

If the Chunk will be enums, nuke all the numbers.
If the Chunk will be numbers, nuke all the enums.