The standard requires overloads of std::to_chars for char and (un)signed
integer types. This means that our constrained function template is
non-conforming, because the difference is observable when using types
that convert to an integer (e.g. wchar_t, which promotes).

As well as defining the deleted bool overload for LWG 3266, replace the
constrained function template with overloads for each type.

	* include/std/charconv (to_chars): Rename to __to_chars_i. Define
	non-template overloads for each signed and unsigned integer type and
	char. Define deleted overload for bool (LWG 3266).
	* testsuite/20_util/to_chars/1_neg.cc: Remove.
	* testsuite/20_util/to_chars/3.cc: New test.
	* testsuite/20_util/to_chars/lwg3266.cc: New test.

From-SVN: r275588