AST walker. Each overridden visit method has full control over what
happens with its node, it can do its own traversal of the node's children,
visit::walk_* to apply the default traversal algorithm, or prevent
deeper traversal by doing nothing.
Note: it is an important invariant that the default visitor walks the body of a function in "execution order" (more concretely, reverse post-order with respect to the CFG implied by the AST), meaning that if AST node A may execute before AST node B, then A is visited first. The borrow checker in particular relies on this property.
Note: walking an AST before macro expansion is probably a bad idea. For instance, a walker looking for item names in a module will miss all of those that are created by the expansion of a macro.
Each method of the