automating RangeTblEntry node support

I have been looking into what it would take to get rid of the
custom_read_write and custom_query_jumble for the RangeTblEntry node
type. This is one of the larger and more complex exceptions left.

(Similar considerations would also apply to the Constraint node type.)

Allegedly, only certain fields of RangeTblEntry are valid based on
rtekind. But exactly which ones seems to be documented and handled
inconsistently. It seems that over time, new RTE kinds have "borrowed"
fields that notionally belong to other RTE kinds, which is technically
not a problem but creates a bit of a mess when trying to understand all
this.

I have some WIP patches to accompany this discussion.

Let's start with the jumble function. I suppose that this was just
carried over from the pg_stat_statements-specific code without any
detailed review. For example, the "inh" field is documented to be valid
in all RTEs, but it's only jumbled for RTE_RELATION. The "lateral"
field isn't looked at at all. I wouldn't be surprised if there are more
cases like this.

In the first attached patch, I remove _jumbleRangeTblEntry() and instead
add per-field query_jumble_ignore annotations to approximately match the
behavior of the previous custom code. The pg_stat_statements test suite
has some coverage of this. I get rid of switch on rtekind; this should
be technically correct, since we do the equal and copy functions like
this also. So for example the "inh" field is now considered in each
case. But I left "lateral" alone. I suspect several of these new
query_jumble_ignore should actually be dropped because the code was
wrong before.

In the second patch, I'm removing the switch on rtekind from
range_table_mutator_impl(). This should be fine because all the
subroutines can handle unset/NULL fields. And it removes one more place
that needs to track knowledge about which fields are valid when.

In the third patch, I'm removing the custom read/write functions for
RangeTblEntry. Those functions wanted to have a few fields at the front
to make the dump more legible; I'm doing that now by moving the fields
up in the actual struct.

Not done here, but something we should do is restructure the
documentation of RangeTblEntry itself. I'm still thinking about the
best way to structure this, but I'm thinking more like noting for each
field when it's used, instead by block like it is now, which makes it
awkward if a new RTE wants to borrow some fields.

Now one could probably rightfully complain that having all these unused
fields dumped would make the RangeTblEntry serialization bigger. I'm
not sure who big of a problem that actually is, considering how many
often-unset fields other node types have. But it deserves some
consideration. I think the best way to work around that would be to
have a mode that omits fields that have their default value (zero).
This would be more generally useful; for example Query also has a bunch
of fields that are not often set. I think this would be pretty easy to
implement, for example like

#define WRITE_INT_FIELD(fldname) \
if (full_mode || node->fldname) \
appendStringInfo(str, " :" CppAsString(fldname) " %d",
node->fldname)

There is also the discussion over at [0] about larger redesigns of the
node serialization format. I'm also interested in that, but here I'm
mainly trying to remove more special cases to make that kind of work
easier in the future.

Any thoughts about the direction?

[0]:
https://www.postgresql.org/message-id/flat/CACxu%3DvL_SD%3DWJiFSJyyBuZAp_2v_XBqb1x9JBiqz52a_g9z3jA%40mail.gmail.com