Parse blast XML output.

If you use a recent version of NCBI BLAST and specify XML output (blastall -m 7), this module should be able to parse the result into a hierarchical BlastResult structure.

While the process may consume a bit of memory, the parsing is lazy, and file sizes of several gigabytes can be parsed (see e.g. the xml2x tool for an example). To parse XML, we use Text.HTML.TagSoup.