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.

lib/cretonne/meta/base/entities.py

@@ -30,3 +30,6 @@ func_ref = EntityRefKind('func_ref', 'An external function.')
 #: A reference to a jump table declared in the function preamble.
 jump_table = EntityRefKind(
         'jump_table', 'A jump table.', default_member='table')
+
+#: A reference to a heap declared in the function preamble.
+heap = EntityRefKind('heap', 'A heap.')