Microcontrollers have a single address space for non-volatile and
volatile memory.

    0x0 - 0x2000 might be the non-volatile Flash
    0x2000 - 0x8000 might be the volatile SRAM

At runtime, the flash can be toggled writable or read-only.

Similarly on desktop PC's, a page of memory can be toggled from
writable to read-only.

If we store a 'const' object in a section of memory that later gets
set read-only, we need to be sure that it is really a const object --
i.e. it can't contain mutable members.

So I've implemented "std::contains_mutable". Here's my compiler patch
for GNU g++:

    https://github.com/healytpk/gcc-thomas-healy/commit/tag_contains_mutable

And here it is tested up on GodBolt:

    https://godbolt.org/z/KosvqTETo

Looking over the recent papers on reflection . . . I think this would
be done in reflection as follows:

    https://godbolt.org/z/4ezv58zxx