Add aid for finding missing dynamic variable declarations

Add aid for finding missing dynamic variable declarations

Find lexical use of variables that are dynamically declared in other
files by recording 'defvar' declarations in files that can be read
in by the compiler in a second compilation.  This is particularly
useful when converting code to use lexical-binding.

The facility is controlled by setting environment variables:

 EMACS_GENERATE_DYNVARS -- set to non-empty to generate a .dynvars file
                           corresponding to each .elc.
 EMACS_DYNVARS_FILE     -- set to the name of a .dynvars file to use
                           as defvar information during compilation,
                           enabling the new warnings.

* lisp/emacs-lisp/bytecomp.el (byte-compile--known-dynamic-vars)
(byte-compile--seen-defvars): New variables.
(byte-compile-warning-types): Add lexical-dynamic warning.
(byte-compile--load-dynvars, byte-compile--warn-lexical-dynamic):
New functions.
* lisp/emacs-lisp/bytecomp.el (byte-compile-file, byte-compile--declare-var)
(byte-compile-lambda, byte-compile-bind): Add dynamic variable loads,
dumps and checks.
* doc/lispref/variables.texi (Converting to Lexical Binding): Document.