The st_future slot of the symtable is not freed by PySymtable_Free()
because it is shared by the symtable and compiling structs in
compiel.c.  Since it is shared, it is explicitly deallocated when the
compiling struct is freed.

of nested functions.  Either is allowed in a function if it contains
no defs or lambdas or the defs and lambdas it contains have no free
variables.  If a function is itself nested and has free variables,
either is illegal.

Revise the symtable to use a PySymtableEntryObject, which holds all
the revelent information for a scope, rather than using a bunch of
st_cur_XXX pointers in the symtable struct.  The changes simplify the
internal management of the current symtable scope and of the stack.

Added new C source file: Python/symtable.c.  (Does the Windows build
process need to be updated?)

As part of these changes, the initial _symtable module interface
introduced in 2.1a2 is replaced.  A dictionary of
PySymtableEntryObjects are returned.