Support heaps with no offset-guard pages.

Also, say "guard-offset pages" rather than just "guard pages" to describe the
region of a heap which is never accessible and which exists to support
optimizations for heap accesses with offsets.

And, introduce a `Uimm64` immediate type, and make all heap fields use
`Uimm64` instead of `Imm64` since they really are unsigned.

lib/codegen/src/ir/trapcode.rs

@@ -17,7 +17,8 @@ pub enum TrapCode {
     /// A `heap_addr` instruction detected an out-of-bounds error.
     ///
     /// Note that not all out-of-bounds heap accesses are reported this way;
-    /// some are detected by a segmentation fault on the heap guard pages.
+    /// some are detected by a segmentation fault on the heap unmapped or
+    /// offset-guard pages.
     HeapOutOfBounds,
 
     /// A `table_addr` instruction detected an out-of-bounds error.