Fix NodeId->HirId lookups by carrying structural context

- record child-slot paths, statement indices, and owner node
IDs in AST span maps via the new child_slot vocabulary and
NodeContextKey
- teach SpanToHirMapper/HirMap to mirror that
context, populate a context_to_hir_map, and fall back to it
before the span-only mapping

refactor: align struct field key trees with bitfield helpers

Author: oinoom

c2rust-refactor/src/ast_manip/mod.rs

@@ -35,7 +35,9 @@ pub use self::load_modules::load_modules;
 pub use self::output_exprs::fold_output_exprs;
 pub use self::remove_paren::remove_paren;
 pub use self::seq_edit::{fold_blocks, fold_modules};
-pub use self::span_maps::{AstSpanMaps, NodeSpan, SpanNodeKind};
+pub use self::span_maps::{
+    child_slot, AstSpanMaps, NodeContextKey, NodeSpan, SpanNodeKind, StructuralContext,
+};
 pub use self::visit::Visit;
 pub use self::visit_node::{visit_nodes, visit_nodes_post, VisitNode};