glist: _Generic wrapper for selective dlist/dclist usage

Hi,

I'd like to track the size of a dlist for data structure validation
purposes[^0]. Normally, one would use a dclist, as this tracks a
count of list elements contained therein, but because this code would
not be called in most normal production builds using a dclist would
waste precious memory.
Manually tracking the length is possible, but tedious, and a local
wrapper around the used dclist/dlist APIs (with different
implementations conditioned with #ifdefs to use the right types) would
also be a significant amount of effort, that'd be duplicated every
time .

Attached is a patch that adds glist_* macros, which wrap several
dlist/dclist_* APIs, so that developers can use dlist/dclist
selectively in different environments, without significant visual
overhead in the code. I'm planning to use this in the Proxy memory
contexts over at [1].

I've considered also adding slist_* to the macros, but I've never
needed selective slist vs dlist/dclist before, so I ignored that list
type for now.

Kind regards,

Matthias van de Meent
Databricks (https://www.databricks.com)

[^0]: This case for builds with MEMORY_CONTEXT_CHECKING, but builds
with USE_ASSERT_CHECKING or WRITE_READ_PARSE_PLAN_TREES -like options
may also want this.

[1]: https://www.postgresql.org/message-id/flat/CAEze2WiPyruOtUOSyRUV8mQssjmYwno0M6hkxC_iUpH-=W8WcA(at)mail(dot)gmail(dot)com