Improve Parser::strip() performance by delaying update of the strip state until the...

Improve Parser::strip() performance by delaying update of the strip state until the end of the parse. This improves performance in two ways: firstly by minimising the size of the state array in the case of large numbers of tags, and secondly by avoiding unnecessary invalidation of the FSS object, when FSS is enabled. The same function is preserved, because a placeholder from one recursion level should never find its way into the output of the same recursion level.