sstring: Add str_(len|begin|end) for std::string
sstring: Add str_(len|begin|end) for std::string

This allows using std::string with make_sstring.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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