Propagate aliased lexical variables in byte compiler

Replace uses of a variable aliasing another variable with that aliased
variable, to allow for variable removal when possible.  This also
enables opportunities for other optimisations.  Example:

 (let ((y x)) (f y)) => (f x)

The optimisation is only performed if both aliased and aliasing
variables are lexically bound.  Shadowing bindings are α-renamed when
necessary for correctness.  Example:

   (let* ((b a) (a EXPR)) (f a b))
=> (let* ((a{new} EXPR)) (f a{new} a))

* lisp/emacs-lisp/byte-opt.el (byte-optimize--aliased-vars): New.
(byte-optimize-form-code-walker): Cancel aliasing upon mutation.
(byte-optimize--rename-var-body, byte-optimize--rename-var): New.
(byte-optimize-let-form): Add the optimisation.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add relevant test cases.

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