include/CompatSet: use std::string_view in struct Feature

include/CompatSet: use std::string_view in struct Feature

Almost all callers pass a C string (in temporary Feature instances),
only `CompatSetHandler::handle()` passes a `std::string` reference.

Allocating a new `std::string` is useless overhead.  This patch
reduces the binary size by 21 kB.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>