Perf tests show this performs better when parsing larger docs; small strings still get the speed boost as the buffer is recycled.

In CharacterReader. This removes the redundant buffer allocation, and simplifies the read.

For small file reads (same as small string test), updated version is ~ 20% faster than original.

Refactored so that it eats until a combinator is seen after non-combinator content, and returns it all.

And corrected unit tests that were incorrectly relying on that behavior.

Note that a leading combinator will combine against the root element, which is either the Document, or the context element.

Fixes jhy#1707