Add yang-model-api model of an XPath expression

Current RevisionAwareXPath is quite deficient in expressing the XPath stored therein. More specifically it requires anyone attempting to use it infer the prefix/namespace mapping and then parse the expression itself.

Instead of going through these hoops, we really should have an object model of the expression, similar to org.jaxen.expr.Expr, which would act as an intermediate representation format, where all prefixes would already be properly resolved so that we are dealing with QNames instead of prefix+localnames.

The parser should also bind function invocations for YANG-defined functions, so that their arguments are resolved to proper types.