ctl::string cleanup

[mrdomino]

We now fully initialize a ctl::string’s memory, so that it is always set to a well-defined value, thus making it always safe to memcpy out of it. This incidentally makes our string::swap function legal, which it wasn’t before. This also saves us a store in string::reserve.

Now that we have made both big_string and smalL_string POD, I believe it is safe to elide the launder calls, and have done so, thus cleaning up a lot of the blob-related code.

I also got rid of set_big_capacity and replaced it with a set_big_string that leaves us in a well-defined state afterwards. This function also is able to be somewhat simpler; rather than delicate bit-twiddling, it just reaches straight into blob and rewrites it wholesale.

Overall, this shaves about 1–2ns off of most benchmarks, and adds 1ns to only one of them - creating a string from a char *.

[mrdomino]

master:

   10.1243 ns 10000000x { ctl::string s; s.append("hello "); s.append("world"); }
   3.79312 ns 1000000x { ctl::string s; for (int i = 0; i < 8; ++i) { s.append('a'); } }
   4.20162 ns 1000000x { ctl::string s; for (int i = 0; i < 16; ++i) { s.append('a'); } }
   4.03643 ns 1000000x { ctl::string s; for (int i = 0; i < 23; ++i) { s.append('a'); } }
   4.89956 ns 1000000x { ctl::string s; for (int i = 0; i < 32; ++i) { s.append('a'); } }
     7.245 ns 1000000x { ctl::string s("hello world"); }
     6.634 ns 1000000x { ctl::string s2(s); }
     8.184 ns 1000000x { ctl::string s("hello world"); ctl::string s2(std::move(s)); }
    13.006 ns 1000000x { ctl::string s("hello world"); ctl::string s2(s); }
     6.717 ns 1000000x { ctl::string s(23, 'a'); }
    16.947 ns 1000000x { ctl::string s(24, 'a'); }
     1.312 ns 1000000x { ctl::string_view s2(s); }

sso-cleanup:

    8.2435 ns 10000000x { ctl::string s; s.append("hello "); s.append("world"); }
   3.67887 ns 1000000x { ctl::string s; for (int i = 0; i < 8; ++i) { s.append('a'); } }
   3.72369 ns 1000000x { ctl::string s; for (int i = 0; i < 16; ++i) { s.append('a'); } }
   3.79352 ns 1000000x { ctl::string s; for (int i = 0; i < 23; ++i) { s.append('a'); } }
   3.66553 ns 1000000x { ctl::string s; for (int i = 0; i < 32; ++i) { s.append('a'); } }
     8.242 ns 1000000x { ctl::string s("hello world"); }
     5.534 ns 1000000x { ctl::string s2(s); }
     7.173 ns 1000000x { ctl::string s("hello world"); ctl::string s2(std::move(s)); }
    10.605 ns 1000000x { ctl::string s("hello world"); ctl::string s2(s); }
     5.856 ns 1000000x { ctl::string s(23, 'a'); }
    14.457 ns 1000000x { ctl::string s(24, 'a'); }
     1.327 ns 1000000x { ctl::string_view s2(s); }