Add remaining std::string_view overloads

[dantargz]

Completing the std::string_view support work defined in #626

The key new functionality is adding a helper function for checking equality between string_view and null terminated c-strings impl::stringview_equal. Using strncmp/wsncmp directly would not work, because that stops comparison at the first null, and the string_view might have a null in the middle -- we do not want a string_view of "n\0pe" to be considered equal to a node name "n" for lookups. string_view has a comparison function that has an overload for const char* (null terminated c-string) that would give the comparison we want. However, that helper works by constructing a string_view from the const char* and then comparing to the new string_view, which would trigger an extra iteration over the const char*.

These overloads are created by copying and pasting the original and substituting the new equality function:

xml_node xml_node::child(string_view_t name) const;
xml_attribute xml_node::attribute(string_view_t name) const;
xml_node xml_node::next_sibling(string_view_t name) const;
xml_node xml_node::previous_sibling(string_view_t name) const;
xml_attribute xml_node::attribute(string_view_t name, xml_attribute& hint) const;

Added some unit tests for child(string_view_t) and attribute(string_view_t). Also added unit tests documenting the behavior of child and attribute lookup when the node/attribute has null name -- searching does not find these nodes, even if an empty argument name is provided.

These other functions also gained overloads, but they just pass the string_view_t to another overloaded function. No new unit tests were added for these functions.

xml_attribute& xml_attribute::operator=(string_view_t rhs);

xml_attribute xml_node::append_attribute(string_view_t name);
xml_attribute xml_node::prepend_attribute(string_view_t name);
xml_attribute xml_node::insert_attribute_after(string_view_t name, const xml_attribute& attr);
xml_attribute xml_node::insert_attribute_before(string_view_t name, const xml_attribute& attr);
xml_node xml_node::append_child(string_view_t name);
xml_node xml_node::prepend_child(string_view_t name);
xml_node xml_node::insert_child_after(string_view_t name, const xml_node& node);
xml_node xml_node::insert_child_before(string_view_t name, const xml_node& node);
bool xml_node::remove_attribute(string_view_t name);
bool xml_node::remove_child(string_view_t name);

xml_text& xml_text::operator=(string_view_t rhs);

Documentation will be updated to cover the new overloads after the opt-in define PUGIXML_STRING_VIEW is removed

[dantargz]

I will re-open after I get a better way to test locally