Move helper methods variable? and blob? to the Field class.

The methods are now implemented by checking the class of the data
type. Also, eliminate the now unneeded class AbstractType.
Handle an optional width specification for the CHAR data type.
Ensure that a width is specified for VARCHAR.
Add support for numeric types: BIT, FLOAT, DOUBLE and DECIMAL.
Add support for date and time types: YEAR, TIME, DATE, DATETIME and
TIMESTAMP.