Unify constants that are equal-including-properties in compiler

This results in a healthy reduction of .elc file size (> 300 KiB saved)
as well as less memory use.

* lisp/emacs-lisp/bytecomp.el (byte-compile-get-constant):
Use 'equal-including-properties' as equivalence relation for all
constants, not just strings.
* etc/NEWS: Mention it.  Strictly speaking it's not an incompatible
change because it doesn't break any earlier promises, but for people
having code that made incorrect assumptions, this provides a useful
explanation.

etc/NEWS

@@ -3049,6 +3049,16 @@ Other functions that use 'aset' to modify string data, such as
 'subst-char-in-string' with a non-nil INPLACE argument, will signal an
 error if called with arguments that would violate these rules.
 
++++
+** More program constants are combined by the compiler.
+The compiler now unifies more constants that are 'equal' for better code
+generation.  This does not affect correct programs but may expose some
+coding mistakes.  For example,
+
+    (eq (cdr '(1 2 3)) '(2 3)))
+
+may return either nil or t.
+
 ** Nested backquotes are not supported any more in Pcase patterns.
 
 ---