Remove XML_PARSE_XINCLUDE. This is only honored by the XML Reader
interface which is now fuzzed in reader.c.
Don't validate in XInclude fuzzer. This doesn't increase coverage after
moving the Reader fuzzer.
Implement a custom mutator that takes a list of fixed-size chunks which
are mutated with a given probability. This makes sure that values like
parser options or failure position are mutated regularly even as the
fuzz data grows large. Values can also be adjusted temporarily to make
the fuzzer focus on failure injection, for example.
Thanks to David Kilzer for the idea.
XIncludes involve XPath processing which can still lead to timeouts when
fuzzing. This will probably take a while to fix. The rest of the XML
parsing code should hopefully run without timeouts now. OSS-Fuzz only
shows a single timeout test case, so separate the XInclude from the core
XML fuzzer.
