Add heaps to the Cretonne IL.

Add preamble syntax for declaring static and dynamic heaps, and update
the langref section on heaps. Add IR support for heap references.

Remove the heap_load and heap_store as discussed in #144. We will use
heap_addr along with native load and store instructions in their place.

Add the heap_addr instruction and document its bounds checking
semantics.

cranelift/filetests/parser/tiny.cton

@@ -136,20 +136,6 @@ ebb0:
 ; nextln: stack_store $v1, $ss10+2
 ; nextln: stack_store $v2, $ss2
 
-; Heap access instructions.
-function %heap(i32) {
-    ; TODO: heap0 = heap %foo
-ebb0(v1: i32):
-    v2 = heap_load.f32 v1
-    v3 = heap_load.f32 v1+12
-    heap_store v3, v1
-}
-; sameln: function %heap(i32) native {
-; nextln: ebb0($v1: i32):
-; nextln:     $v2 = heap_load.f32 $v1
-; nextln:     $v3 = heap_load.f32 $v1+12
-; nextln:     heap_store $v3, $v1
-
 ; Memory access instructions.
 function %memory(i32) {
 ebb0(v1: i32):