Mark std.string.toStringz as @trusted

`toStringz(const(char)[])` uses system functions `core.stdc.string.strlen`, `core.stdc.string.memcmp`. `toStringz(string)` uses a system function `std.exception.assumeUnique`.
But in both cases, we can verify that they can be trusted.

I also mark a unittest for them as `pure nothrow`.