Need JSONiq-based JSound implementation

[Chris Hillery]

Need the ability to process .jsd files and verify that a JSONiq item validates according to a .jsd schema. For now I believe we only need a jnlib: function which does this; no need to update the validate {} expression or anything internal like that. It should be possible to do what we need entirely in JSONiq without C++, although if there are reasons to use C++ that's fine too. This should be core functionality.