Preliminary support for future nested scopes

compile.h: #define NESTED_SCOPES_DEFAULT 0 for Python 2.1
           __future__ feature name: "nested_scopes"

symtable.h: Add st_nested_scopes slot.  Define flags to track exec and
    import star.

Lib/test/test_scope.py: requires nested scopes

compile.c: Fiddle with error messages.

    Reverse the sense of ste_optimized flag on
    PySymtableEntryObjects.  If it is true, there is an optimization
    conflict.

    Modify get_ref_type to respect st_nested_scopes flags.

    Refactor symtable_load_symbols() into several smaller functions,
    which use struct symbol_info to share variables.  In new function
    symtable_update_flags(), raise an error or warning for import * or
    bare exec that conflicts with nested scopes.  Also, modify handle
    for free variables to respect st_nested_scopes flag.

    In symtable_init() assign st_nested_scopes flag to
    NESTED_SCOPES_DEFAULT (defined in compile.h).

    Add preliminary and often incorrect implementation of
    symtable_check_future().

    Add symtable_lookup() helper for future use.

Include/compile.h

@@ -7,6 +7,9 @@
 extern "C" {
 #endif
 
+#define NESTED_SCOPES_DEFAULT 0
+#define FUTURE_NESTED_SCOPES "nested_scopes"
+
 /* Bytecode object */
 typedef struct {
     PyObject_HEAD