XPath for Swift

A wrapper around libxml2’s XPath facilities.

SPM and Carthage compatible.

For SPM on macOS, because the headers for the system libraries are no longer installed in the system, one (usually) has to launch the Swift commands with the following options:

-Xcc -I"`xcode-select -p`/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/libxml2"

I did not find a way to circumvent the problem (on macOS 10.14.4 w/ Xcode 10.2, without installing macOS_SDK_headers_for_macOS_10.14 which is deprecated). If you have a suggestion, feel free to help out!