...in the <flux/core/numeric.hpp> header

This PR does a whole lot of things.

Firstly, we define some concepts for integers. Specifically, the `flux::num::integral` concept is satisfied by any `std::integral` type *except* `bool`, `char`, `wchar_t` and the various `charN_t`s. We also have corresponding `signed_integral` and `unsigned_integral` concepts.

Next, we define some functions which perform *unchecked* integer operations, namely:

 * `unchecked_add`
 * `unchecked_sub`
 * `unchecked_mul`
 * `unchecked_div`
 * `unchecked_mod`

These work call the built-in operators (and so can cause UB for signed types), but require both arguments to be the same type, and cast the result back to their argument type -- that is, there is no promotion, so `unchecked_add(short, short)` returns a `short`, not an `int`. The intention is that these can be used in places where signed UB can allow extra optimisations, and explicitly acknowledge that you're doing something dangerous.

Next is a set of wrapping functions:

* `wrapping_add`
* `wrapping_sub`
* `wrapping_mul`

These work by casting their arguments to an unsigned type, performing the operation, and casting back to the starting type. They never cause UB, and can be used to specifically document that you want wrapping semantics.

Next is a set of functions which check whether overflow occurred:

 * `overflowing_add`
 * `overflowing_sub`
 * `overflowing_mul`

These return a `(T, bool)` pair which safely performs the operation (as if by wrapping) and reports whether overflow occurred. They use compiler builtins in GCC and Clang. These work for unsigned types as well as signed types, so you can test whether your size_t overflowed.

Then we have a set of checked arithmetic functions:

* `checked_add`
* `checked_sub`
* `checked_mul`
* `checked_div`
* `checked_mod`

These raise a `flux::runtime_error` if overflow occurs (or division by zero for the last two functions), regardless of the compiled checking policy.

Finally, we have

* `add`
* `sub`
* `mul`
* `div`
* `mod`

These perform overflow/divide-by-zero checks according to the configured policies. By default, they trap on overflow in debug mode or wrap in release mode.

Phew!

This adds
 * num::unchecked_shl
 * num::unchecked_shr
 * num::checked_shl
 * num::checked_shr
 * num::shl
 * num::shr

Which perform left and right bitshift operations either explicitly without undefined behaviour checking, or which check whether the shift amount is out of bounds

I got them the wrong way round

Results seem to be too inconsistent on CI

...where it's used.

One day we might make this public again, but for now let's just have it as an implementation detail.

Originally the idea was that I wanted to allow user-defined types, e.g `enum class my_int : int {}` to be declared as "extended integers", and be used with the Flux numeric functions.

But YAGNI (at least for now) so let's keep things simple

Use the new C++20 std::in_range(), which seems to generate better code with GCC (and exactly the same with Clang)

This function either calls `num::checked_cast` or `num::unchecked_cast` depending on the currently configured integer cast policy. In checked mode, lossy casts -- that is, those that result in a change in value -- will raise a runtime error, whereas in unchecked mode these will silently use the incorrect value.

...rather than checked_cast.

...in function arguments, because I don't think flux::chunk(seq, 'a') is something we should allow.

...at least when using integral types.

It seems like std::invoke() in libc++18 (but not previous versions) doesn't like default function parameters in certain contexts.

Reported as llvm/llvm-project#106428

Specifically,

 * num::unchecked_neg
 * num::wrapping_neg
 * num::overflowing_neg
 * num::checked_neg
 * num::neg

Each call to `xxx_neg(val)` is semantically the same as `xxx_sub(0, val)`, but is shorter to spell and may generate better code.

It seems like MSVC doesn't consider -INT_MIN to overflow even in constexpr mode.

This version generates better code with Clang and GCC anyway, so that's good.