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/src/legalizer/mod.rs

@@ -22,9 +22,11 @@ use bitset::BitSet;
 
 mod boundary;
 mod globalvar;
+mod heap;
 mod split;
 
 use self::globalvar::expand_global_addr;
+use self::heap::expand_heap_addr;
 
 /// Legalize `func` for `isa`.
 ///