Improve support for type information

[flbulgarelli]

It would be nice that some common types would be reifyed:

data MuType = GenericNumberType 
                      | GenericDecimalType 
                      | IntType 
                      | LongType 
                      | FloatType 
                      | DoubleType 
                      | BooleanType
                      | StringType 
                      | UnitType
                      | OtherType Identifier