This patch removes 'label_value' from lang_identifier, shrinking it from 
72 to 64 bytes (on 64-bit machine).   We replace this by augmenting the 
already used per-function named_labels hash table.  This is a major win, 
because labels are extremely rare and there are many identifiers.  We 
also shring the binding structure by a pointer, as the shadowed_labels 
list goes away.

The slight difficulty is with the declared label extension.  These have 
a regular scope and can hide an outer binding.  We stash the outer 
binding in the named_label_entry and restore it when popping that scope.

Applying to trunk

nathan
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Nathan Sidwell