Optimize memory in span_map

[callegarimattia]

          @callegarimattia I identified one place we could optimise. In Span Map instead of storing all "HotCharacters" in the Span, we can only store the str values. This way we will save some memory.

Can you create an issue after merging this PR and we could work on it?

The function in question is: extract_spans - which you already flagged to be deprecated

Originally posted by @krishnasism in https://github.com/weareprestatech/hotpdf/issues/34#issuecomment-1892892533