sstring: mark str() and methods using it as noexcept
sstring: mark str() and methods using it as noexcept

Note that we assume that std::basic_string_view<char_type>(str(), size())
won't throw, although it is not specified as noexcept in
https://en.cppreference.com/w/cpp/string/basic_string_view/basic_string_view
at this time (C++20).

This is similar to std::string operator std::basic_string_view:
https://en.cppreference.com/w/cpp/string/basic_string/operator_basic_string_view
that is specified as noexcept too.

Plus, std::basic_string_view's respective constructor is specified
as noexcept in /usr/include/c++/10/string_view

Test: unit(dev)
Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20200809063039.652669-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar,scylladb/seastar,avikivity/seastar,dreamsxin/seastar,syuu1228/seastar,cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar,syuu1228/seastar