remove_whitespace_children

In HTML, certain elements such as <ul> cannot have any text nodes in them, and any whitespace is ignored, see stackoverflow.com/questions/2161337/can-we-use-any-other-tag-inside-ul-along-with-li/60885802#60885802.

A similar concept applies to OurBigBook, e.g.:does not parse as:but rather as:because the content argument of ul is marked with remove_whitespace_children and automatically removes any whitespace children (such as a newline) as a result.

This also applies to consecutive sequences of auto_parent macro property macros, e.g.:also does not include the newline between the list items.

The definition of whitespace is the same as the ASCII whitespace definition of HTML5: \r\n\f\t.