Remove unsafe CString::yolo from ffi

CString::yolo was still used in the ffi, this was an unsafe
transitional thing.  To remove it there were two choices: 1. make
errors in creating CStrings hard errors or 2. try and be as lenient as
possible.  Given the to_string_lossy() convention adopted in the ffi
this choose the lenient option and simply skips over embedded null
bytes, leaving the rest of the strings intact.

Thus now CString::new_lossy().  It's only used for .strdup() however
so no longer a public trait.

This also cleans up the public visibility of things in the strings.rs
file:

- Rename StrExt/OptStrExt traits to what they actually do: provide
  .strdup() -> Strdup/OptStrdup.

- dc_strdup() should be an implementation detail, replace all usages
  with Strdup.strdup() method.

- Only allow visibility inside the crate for all things.

- Reduce visibility to only the module for things not used in lib.rs.

tests/stress.rs

@@ -44,9 +44,9 @@ fn stress_functions(context: &Context) {
     // assert!(dc_is_configured(context) != 0, "Missing configured context");
 
     // let setupcode = dc_create_setup_code(context);
-    // let setupcode_c = CString::yolo(setupcode.clone());
+    // let setupcode_c = CString::new(setupcode.clone()).unwrap();
     // let setupfile = dc_render_setup_file(context, &setupcode).unwrap();
-    // let setupfile_c = CString::yolo(setupfile);
+    // let setupfile_c = CString::new(setupfile).unwrap();
     // let mut headerline_2: *const libc::c_char = ptr::null();
     // let payload = dc_decrypt_setup_file(context, setupcode_c.as_ptr(), setupfile_c.as_ptr());