Add remaining std::string_view overloads

[dantargz]

Adding overloads for string_view to core public APIs as described in #626

The most material change 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 named "n" for lookups. string_view has a compare 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 that new string_view, which would trigger an extra iteration over the const char*. The existing helper strequalrange also cannot be used: if the string_view is "n\0pe" and the test name is "n", the n and null characters will match, and then the function will attempt to read past the end of the allocation of the test name.

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

[zeux]

Thanks! Would you mind also replicating dom_node_attribute_hinted test for string_view, as this is the only function with non-trivial logic that had to be copied? The code changes look good to me. There might be some missing coverage to sort out in the future, but the other functions are trivial so they should be correct.

[dantargz]

I will add the clone test for dom_node_attribute_hinted. I am also going to add simple unit tests for the rest of the functions so that there is basic coverage for everything, let me know if you would prefer that the tests be added in a separate PR to keep the size of this PR low / reduce churn.

[zeux]

As long as the changes are test-only it's fine to add in this PR.

[zeux]

Merged, thanks a lot!