Implement and-let*

This also includes changes to if-let and when-let.  The single tuple
special case is ambiguous, and binding a symbol to nil is not as
useful as binding it to its value outside the lexical scope of the
binding.  (Bug#28254)
* etc/NEWS: Mention.
* lisp/emacs-lisp/subr-x.el (internal--listify):
(internal--build-binding-value-form): Extend to account for
solitary symbols and (EXPR) items in binding varlist.
(if-let*, when-let*): Nix single tuple case and incumbent
bind-symbol-to-nil behavior.
(and-let*): New macro.
(if-let, when-let): Mark obsolete.  Redefine in terms of if-let*, so
they implicitly gain the new features without breaking existing code.
* test/lisp/emacs-lisp/subr-x-tests.el: Adjust tests for: lack of
single-tuple special case, lack of binding solitary symbols to nil,
and the introduction of uninterned symbols for (EXPR) bindings.  Add
SRFI-2 test suite adapted to Elisp.

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