char8_t overloads not implemented

[rolandreichwein]

In include/boost/range/as_literal.hpp, the C++20 char8_t overloads are not implemented, in contrast to the already existing char16_t and char32_t versions. This leads to e.g.

boost::algorithm::starts_with(u8".git", u8".") == false

but only for C++20, and with u8, resp. char8_t.

[rolandreichwein]

Please consider the attached patch that fixes it for me.
(.zip since interface didn't accept .patch directly)

boost-range.zip