symtable.h, so that they can be used by external module.

Improve error handling in symtable_enter_scope(), which return an
error code that went unchecked by most callers. XXX The error handling
in symtable code is sloppy in general.

Modify symtable to record the line number that begins each scope.
This can help to identify which code block is being referred to when
multiple blocks are bound to the same name.

Add st_scopes dict that is used to preserve scope info when
PyNode_CompileSymtable() is called.  Otherwise, this information is
tossed as soon as it is no longer needed.

Add Py_SymtableString() to pythonrun; analogous to Py_CompileString().