P0052R10 Generic Scope Guard and RAII Wrapper for the Standard Library

Substantial editorial changes have been applied; see the Editor's Report
for details. In short, the paper is written against the IS WD, but it is
being applied to the Fundamentals TS. A summary of the changes:

 * C++20-style Mandates/Expects/Contraints elements have been
   rephrased as Requires/Remarks.
 * Edits to change the wording of the IS WD [requirements] have been
   replaced with new wording in [mods]; "Remarks:" changed to "Throws:".
 * Namespaces and header names have been changed to include “experimental”.
 * Requirements changed from Cpp17Foo to Foo.
 * The placeholder class name has been changed from "scope_guard" to "scope-guard".
 * Stable labels have been changed.
 * To avoid hanging paragraphs, non-normaive, introductory wording has been dropped/moved.
 * The description "factory function" has been renamed to "unique_resource creation".
 * Introductory words "The class template" have been prepended to 7.5.2p1.
 * The three separate constructor specifications for scope_exit, scope_fail, and scope_success
   have been merged into a single specification for the placeholder scope-guard.
 * The phrase "Throws: Nothing unless the initialization of exit_function throws" has been
   replaced with "Throws: Any exception thrown during the initialization of exit_function".