add valgrind_stack_change

Unlike valgrind_stack_register, this won't automatically swap
if `start > end`. It's probably wiser to pretend it doesn't swap at all.

src/valgrind.c

@@ -7,6 +7,10 @@ valgrind_stack_id_t valgrind_stack_register(const void *start, const void *end)
   return VALGRIND_STACK_REGISTER(start, end);
 }
 
+void valgrind_stack_change(valgrind_stack_id_t id, const void *start, const void *end) {
+  VALGRIND_STACK_CHANGE(id, start, end);
+}
+
 void valgrind_stack_deregister(valgrind_stack_id_t id) {
   VALGRIND_STACK_DEREGISTER(id);
 }

src/valgrind.rs

@@ -1,6 +1,6 @@
 #![allow(non_camel_case_types)]
 //! In order for Valgrind to keep track of stack overflows and such, it needs
-//! a little help. That help unfortunately comes in the form of a pair of C
+//! a little help. That help unfortunately comes in the form of a set of C
 //! macros. Calling out to un-inlineable C code for this is pointlessly slow,
 //! but that's the way it is for now.
 
@@ -9,9 +9,14 @@ extern "C" {
   #[link_name = "valgrind_stack_register"]
   /// Register a stack with Valgrind. Returns an integer ID that can
   /// be used to deregister the stack when it's deallocated.
-  /// `start < end`, though Valgrind will happily accept either.
+  /// `start < end`.
   pub fn stack_register(start: *const u8, end: *const u8) -> stack_id_t;
 
+  #[link_name = "valgrind_stack_change"]
+  /// Change the size or location of a stack registered with Valgrind.
+  /// `start < end`.
+  pub fn stack_change(id: stack_id_t, start: *const u8, end: *const u8) -> stack_id_t;
+
   #[link_name = "valgrind_stack_deregister"]
   /// Deregister a stack from Valgrind. Takes the integer ID that was returned
   /// on registration.