Adapt `fsyspath` for C++20 conventions.

In C++20, `std::filesystem::u8path()` (that accepts a UTF-8 encoded
`std::string` and returns a `std::filesystem::path`) is deprecated. Instead,
to engage UTF-8 coding conversions, we're supposed to pass the `path`
constructor a `std::u8string`, i.e. a `std::basic_string<char8_t>`. Since
`char8_t` is a type distinct from both `char` and `unsigned char`, we must Do
Something to pass a UTF-8 encoded `std::string` into `std::filesystem::path`.

To avoid copying characters from a `std::string` into a temporary
`std::u8string` and from there into the `std::filesystem::path`, make a
`boost::transform_iterator` that accepts a `std::string_view::iterator` and
adapts it to dereference `char8_t` characters. Make `fsyspath(std::string_view)`
engage the base-class constructor accepting (iterator, iterator), adapting
`string_view::begin()` and `end()` to deliver `char8_t` characters.

Use the same tactic for `fsyspath::operator=(std::string_view)`, explicitly
calling `std::filesystem::path::assign()` with the adapted iterators.

To resolve ambiguities, provide both constructors and assignment operators
accepting `(const std::string&)` and `(const char*)`, explicitly converting
each to `std::string_view`.

At the same time, `std::filesystem::path::u8string()` now returns
`std::u8string` rather than `std::string`. Since `std::filesystem::path`
delivers only that `std::u8string` rather than iterators into its internal
representation, we can't avoid capturing it and copying to the returned
`std::string`.

Remove explicit `.u8string()` calls from a few existing `fsyspath` instances,
now that `fsyspath` supports implicit conversion to `std::string`.

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