Speed optimizations for: 39170: jsoup:XmlFuzzer: Timeout in XmlFuzzer

[jhy]

https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=39170

These are mostly in the HTML Parser (via the XML Decl parsing - good to fix sometime but doesn't really come up outside of the fuzzer).

[jhy]

Initially: 0.48 seconds per iter

With the following changes, improves to 0.19 per iter and so within our bounds:

Change	Note
4b46397	Short circuit scans for custom elements
d3f4e31	Flyweight (memoize) the Tag.valueOf(custom) per document scan, so reduces GC
a8df71b	Limit stack depth scanning for DD/DT tags (was unbounded)
e4ae6fa	Only need to foster if the current stack element is a table type