[PATCH, libstdc++] Two parts of C++20 p1032...

[Smith-Rowland, Edward M]

These patches are modest reworkings of previous patches.

The patch for char_traits was pretty much approved already.  But in response to a comment from François I at least added __copy_backwards, and used ptrdiff_t instead of size_t parameters and I put all the mem* wrappers in a __detail namespace. I didn't change some names. I know Jonathan wanted a bikeshed discussion.

For the constexpr iterators I was asked to simplify the testsuite. Done.

These would finish p1032 except for std::string::copy which I think should wait behind the rest of constexpr string.

Another thing I need to do is respond to the SG-10 changes for the p1032 macros.  There's one thing that bothers me still in light of the policy that these macros are names __cpp_lib_constexpr_HEADER: We sill have __cpp_lib_constexpr_algorithmS in <algorithm>.

OK if these pass final retesting?
-------------- next part --------------
A non-text attachment was scrubbed...
Name: p1032_char_traits.CL
Type: text/x-opencl-src
Size: 880 bytes
Desc: p1032_char_traits.CL
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20191210/d3132543/attachment.bin>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: p1032_char_traits.patch
Type: text/x-patch
Size: 22529 bytes
Desc: p1032_char_traits.patch
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20191210/d3132543/attachment-0001.bin>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: p1032_insert_iterators.CL
Type: text/x-opencl-src
Size: 778 bytes
Desc: p1032_insert_iterators.CL
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20191210/d3132543/attachment-0002.bin>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: p1032_insert_iterators.patch
Type: text/x-patch
Size: 14574 bytes
Desc: p1032_insert_iterators.patch
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20191210/d3132543/attachment-0003.bin>