Fix build warnings/errors in C++20 (#2883)

[berhoel]

FlashString static initialisation

The Object copy and move constructors are deleted as a precaution as these objects should not be copied. C++20 doesn't like this so they're only compiled for C++17 and earlier.

std:is_pod deprecated

Use std::is_standard_layout instead (only one instance to correct).

Volatile

C++20 deprecates many uses of volatile which may be implemented in an ambiguous (non-deterministic) way. Largely these uses are concerned with threaded applications. A general web search turns up plenty of articles on this.

One argument is related to code like this:

volatile int num;
++num; // Is this atomic? Or a read, followed by load?

So we should identify all such cases and code them more explicitly.

The FIFO/FILO classes define numberOfElements as volatile but I can see no reason why this needs to be the case.

Volatile bitwise operations

C++23 will, it seems, de-deprecate use of volatile for bitwise operations. This is discussed here https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2327r1.pdf. See also earlephilhower/arduino-pico#1379

These are genuine uses of volatile as values are used both in interrupt and task context. Some of these actually need to be atomic, and the assumption is probably that x |= n does that. Hence the ambiguity. These are the core Sming files affected:

• Sming/Arch/Esp8266/Components/driver/i2s.cpp
• Sming/Arch/Esp8266/Components/spi_flash/iram_precache.cpp
• Sming/Arch/Esp8266/Components/gdbstub/appcode/gdb_hooks.cpp
• Sming/Arch/Esp8266/Components/gdbstub/gdbstub.cpp
• Sming/Arch/Esp8266/Components/gdbstub/gdbuart.cpp
• Sming/Arch/Esp8266/Core/Digital.cpp

There will be many others in libraries.

Solution: Rather than attempting to 'correct' the code and introduce bugs - risks identified in the above paper - it is safer to just silence this particular warning. This has been done in the main Sming build.mk file so it's easy to revert if required. This is the volatile warning which covers other uses as well - future GCC revisions may add some granularity to this.

FIFO / FILO uses int instead of unsigned

No warnings but using int for the count makes no sense. Not related to C++20 but since we're modifying the classes anyway may as well do it here.