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.

cranelift/docs/heapex-sm32.clif

@@ -3,7 +3,7 @@ test verifier
 function %add_members(i32, i32 vmctx) -> f32 baldrdash {
     gv0 = vmctx
     gv1 = load.i32 notrap aligned gv0+64
-    heap0 = static gv1, min 0x1000, bound 0x10_0000, guard 0x1000
+    heap0 = static gv1, min 0x1000, bound 0x10_0000, offset_guard 0x1000
 
 ebb0(v0: i32, v5: i32):
     v1 = heap_addr.i32 heap0, v0, 1