XML is slow to parse and strings inside the document cannot be memory mapped as
they do not have a trailing NUL char. The libxmlb library takes XML source, and
converts it to a structured binary representation with a deduplicated string
table -- where the strings have the NULs included.

This allows an application to mmap the binary XML file, do an XPath query and
return some strings without actually parsing the entire document. This is all
done using (almost) zero allocations and no actual copying of the binary data.

As each node in the binary XML file encodes the 'next' node at the same level
it makes skipping whole subtrees trivial. A 10Mb binary XML file can be loaded
from disk *and* queried in less than a few milliseconds.