Here is a new version of this patch. What was changed:
* I rebased code against the latest version of master and adapted recent
changes about the expression execution
* Several names (functions and related pg_type column) were changed
* A new oid function field was introduced to handle nested assignment situation
* I updated the documentation for patch
* `MAXDIM` was replaced by `MAX_SUBSCRIPT_DEPTH`
* I returned one `SubscriptingRef` for both fetch & assign operations, since
there is no real readability improvements at this point (they're already
separated at the time of evaluation, and besides the evaluation code fetch &
assign are handled almost identically).
