New interface for Pratt

[Whiteknight]

Consider an interface like this for setting up a TDOP with Pratt:

var parser = OperatorPrecidence(c => c
    .Level(l => l
        .Left(Number())
    )
    .Level(l => l
        .Left(Match('-'), (ctx, state, token) => { ... })
    )
    .Level(l => l
        .Right(Match('+'), (ctx, state, token) => { ... })
        .Right(Match('-'), (ctx, state, token) => { ... })
    )
    .Level(l => l
        .Right(Match('*'), (ctx, state, token) => { ... })
        .Right(Match('/'), (ctx, state, token) => { ... })
    )
    .Level(l => l
        .RightAssociative(r => r
            .Right(Match('.'), (ctx, state, token) => { ... })
       )
    )
);

With a setup like this, we don't have to manually change precedence levels every time we want to insert something into the table, we just add a new .Level() call and everything just works. Need to do some work on the Left/Right associativity though.