Making position a type member in parsers.scala

Hi,

The parser combinators works wonderfully, except for a minor tweak I'd like to add: I would like to constrain the type of position data structure in my parser pipeline.

At the moment, parsers.scala will use the base type Position, and leave the specific position type to Readers. The downstream operation will need to perform pattern matching to cast the position data into specific type, e.g. OffsetPosition. This gets tedious if I already know that my parser will only use one specific Position type

I propose to make position a type member in a similar fashion to Elem and Input. Existing code base can be kept in-tact by adding

type Pos = Position. 

If needed, a user can constrain the position type to be more specific by overriding the type member;
e.g.

override type Pos = OffsetPosition

I will send out a pull request on the proposed change. Please take a look.

Start in parsers.scala, comparing the existing Position handling with the Elem and Input type members and the Readers that provide specific positions such as OffsetPosition. The change is done when Pos defaults to Position for existing code and can be overridden with a more specific position type without requiring downstream casts.