Add extracting APIs that reset span bounds

[natecook1000]

This change adds three ParserSpan.extracting APIs that yield new ParserSpan instances that have their internal span boundaries shrunk to fit, instead of just insetting the boundaries at the ParserSpan level. An extracted span can be passed off to a parsing function without worry that an absolute seek will go beyond the visible boundaries of the span.

Checklist

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[natecook1000]

Re: naming, I went with extract(...) here (instead of the extracting name that the span types use) because these new methods mutate the underlying ParserSpan. The extract methods are intended as counterparts to sliceSpan and sliceRange, so they use the same part of speech.

[numist]

  /// An _extracted_ parser span doesn't retain information about the bounds of
  /// the original span…

Love it. This interface is going to be a big improvement for container formats (or formats that use framing), where child parsers should be restricted to their own range.