Use SYMBOL_VALUE/SET_SYMBOL_VALUE.

Use SYMBOL_VALUE/SET_SYMBOL_VALUE.
(Qcyclic_variable_indirection): New variable.
(Fkeywordp): Check for internedness differently.
(Fmakunbound): Simplify the test if symbol is a constant.
(indirect_variable, Findirect_variable): New functions.
(swap_in_symval_forwarding): If SYMBOL is an alias, use the
aliased symbol.
(let_shadows_buffer_binding_p): Check for variable aliases.
(set_internal): Simplify the test if SYMBOL is a constant.  If
SYMBOL has a buffer-local value and is an alias, use the aliased
symbol instead.
(syms_of_data): Initialze Qcyclic_variable_indirection and defsubr
Sindirect_variable.