Add heap_addr custom legalization.

The expansion of a heap_addr instruction depends on the type of heap and
its configuration, so this is handled by custom code.

Add a couple examples of heap access code to the language reference
manual.

lib/cretonne/meta/base/legalize.py

@@ -46,6 +46,7 @@ expand = XFormGroup('expand', """
 
 # Custom expansions for memory objects.
 expand.custom_legalize(insts.global_addr, 'expand_global_addr')
+expand.custom_legalize(insts.heap_addr, 'expand_heap_addr')
 
 x = Var('x')
 y = Var('y')