Refactor pattern matching to be static & stand alone

[atifaziz]

This PR refactors pattern matching from the virtual Match/SingleMatch dispatches into a stand-alone static method with a consolidated implementation. This moves away from an object-oriented approach. The rationale for the new approach is that all the patterns are essentially a closed set of discriminated unions. The Pattern and subclasses are internal implementation details. They are not designed for extensibility so the open-close principal doesn't come into play here. Having a consolidated implementation that pattern-matches through a switch table simplifies things greatly (including for understanding). For example, all branch patterns now just boil down to this:

class Required : BranchPattern
{
    public Required(params Pattern[] patterns) : base(patterns) { }
}

class Optional : BranchPattern
{
    public Optional(params Pattern[] patterns) : base(patterns) { }
}

// Marker/placeholder for [options] shortcut.
class OptionsShortcut : Optional { }

class Either : BranchPattern
{
    public Either(params Pattern[] patterns) : base(patterns) { }
}

class OneOrMore : BranchPattern
{
    public OneOrMore(params Pattern[] patterns) : base(patterns) { }
}

A less obvious motivation is that this simplification will potentially also help with static source code generation being explored in #77. While this can't be proven, the PR hopefully has merits on its own.

[voieducode]

Much easier to understand now that it is in a single place!