[XQuery] IBM-XQ-016: Add context item to static context

(IBM-XQ-016) Section 2.1.1, Static Context: Static type analysis of an 
expression that contains a "dot" (such as ". + 1" or "substr(., 1)") 
depends on the static type of the context item. But there is no component 
in the static context for the static type of the context item. Should we 
add such a component and allow implementations to initialize it (since 
they are allowed to initialize the context item)?

--Don Chamberlin