GSL 3, the next major version of GSL after the one we're using, replaced
their local implementation of `span` with one that more closely mimics
C++20's span. Unfortunately, that is a breaking change for all of GSL's
consumers.
This commit updates our use of span to comply with the new changes in
GSL 3.
Chief among those breaking changes is:
* `span::at` no longer exists; I replaced many instances of `span::at`
with `gsl::at(x)`
* `span::size_type` has finally given up on `ptrdiff_t` and become
`size_t` like all other containers
While I was here, I also made the following mechanical replacements:
* In some of our "early standardized" code, we used std::optional's
`has_value` and `value` back-to-back. Each `value` incurs an
additional presence test.
* Change: `x.value().member` -> `x->member` (`optional::operator->`
skips the presence test)
* Change: `x.value()` -> `*x` (as above)
* GSL 3 uses `size_t` for `size_type`.
* Change: `gsl::narrow<size_t>(x.size())` -> `x.size()`
* Change: `gsl::narrow<ptrdiff_t>(nonSpan.size())` -> `nonSpan.size()`
during span construction
I also replaced two instances of `x[x.size() - 1]` with `x.back()` and
one instance of a manual array walk (for comparison) with a direct
comparison.
NOTE: Span comparison and `make_span` are not part of the C++20 span
library.
Fixes#6251
54a7fce3e0