It turns out that it's not very useful to keep the sign
value as a separate argument in the pql.Decimal struct.
This commit incorporates it into Value, and makes Value
an `int64` (for some bone-headed reason I had made it a
`uint32` before which is just dumb).
This commit introduces a new type: pql.Decimal
We use that instead of float64 in order to ensure
that the string representation is consistent.
One unfortunate discovery during implementation is
that the RowAttrs and ColAttrs support floats, and
the PEG file was treating them as such. So I had
to split the PEG definitions into float-specific
items and decimal-specific items.
