Safe casting of PgNode values with bindgen tag discovery

[cbandy]

Postgres implements various node types to represent parse, plan, and executor trees. It uses tagged polymorphism to emulate object-oriented inheritance in C: every node struct begins with a NodeTag field that identifies its type at runtime. Pointers to these nodes are accessed generically as Node * and queried using the IsA macro. The hierarchy includes intermediate base types, such as Expr, which are inherited by concrete types like Const and Var.

This PR introduces type-safe node casting (both upcasting and downcasting) via the PgNode trait. Safe casts are statically generated with the trait implementations emitted by pgrx-bindgen. Each node type may be cast to any of its "parent" types, all the way up to Node.

The PgNode trait is extended with:

• A constant CAST_TAGS slice containing the valid tags for the type and all of its descendants. This is efficient because the hierarchy is shallow and wide. The vast majority of types have four or fewer tags in CAST_TAGS.
• A try_cast associated function and a try_as method check CAST_TAGS before performing an unsafe pointer cast. The lookup is a simple linear scan and relies on compiler and CPU optimizations.
• A node_tag method that returns the tag and an is_a method that behaves like the IsA macro.

Postgres 13 and 14 use the Value struct to represent multiple possible value types, including integer, float, and string. Each type has its own NodeTag, but they can't be automatically associated with the struct using the information in the bindgen bindings. This hard-codes the associations just before emitting all the PgNode implementations.

Postgres 15 introduced separate structs for each value type. These fit nicely into the automatic struct-to-tag scheme, but we weren't handling the ValUnion union Postgres uses to represent these multiple types in a single Node. This extends the detection of parent-child types to implement PgNode for union nodes.

See: #1048

[cbandy]

This is ready! The diff --stat is huge because I included the changes to the generated files to aide in review:

git diff --stat -- pgrx-pg-sys/src/include
 7 files changed, 11323 insertions(+), 3191 deletions(-)

The code changes are small-ish:

git diff --stat -- pgrx-bindgen pgrx-pg-sys/src/node.rs
 2 files changed, 404 insertions(+), 153 deletions(-)

[eeeebbbbrrrr]

I feel like this ought to allow casing to pg_sys::Node too, yeah? I don't see a need to include that in every CONST_TAGS definition, but hardcoding it here would be alright as an else if condition

[cbandy]

Yep, done. Rather than include every tag on Node, the code generation overrides this function for Node so every node can cast to it: build.rs#L600-L610

#[inline]
fn try_cast<T: pg_sys::PgNode>(node: &T) -> Option<&Self> {
    Some(node.as_node())
}

[cbandy]

This function and the try_as method kinda swap arguments and types:

pg_sys::Expr::try_cast(&some_pg_node) -> Option<&pg_sys::Expr>
some_pg_node.try_as::<pg_sys::Expr>() -> Option<&pg_sys::Expr>

pg_sys::Node::try_cast(&some_pg_node) -> Option<&pg_sys::Node>
some_pg_node.try_as::<pg_sys::Node>() -> Option<&pg_sys::Node>
some_pg_node.as_node() -> &pg_sys::Node