Implement PgNode for Value and ValUnion types

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.

Author: cbandy