Improve date and datetime parsing

[bicycle1885]

The current encoding of token kinds can be improved by using a more efficient encoding that can encode all the data types including dates and datetimes into four bits. For example:

0000: String
0001: Integer
0010: Float
0011: Integer or Float
0100: Bool
0101: Quoted string
0110: Date
0111: DateTime
1000: Missing

And this still has room to encode more types. For example, it can distinguish "NA" and empty missings.