A variant of Apply() that works with any TextParser<TextSpan>

[nblumhardt]

While working through #32 I couldn't find an idiomatic/allocation-free way for a text parser to break up a number like 20180521 into numeric parts. This led me to a bit of a general pattern, like this:

var input = new TextSpan("1234");
var twodigits = Span.Length(2).Apply(Numerics.IntegerInt32);
var result = twodigits(input);
Assert.Equal(12, result.Value);

This variant of Apply() makes it possible to split the tasks of delimiting the span to be parsed (Span.Length(2)) from its translation into the value type (here, taking 12 and parsing an int).

Not sure how frequently this will get used - definitely quite pleasant for length-oriented formats like this one - but it's a nice complement to the existing token-oriented Apply() combinator, so might prove fruitful and/or stimulating to include.