fix: String may be written twice in one codepath

[EmilyMatt]

This was discovered by running miri on simd-json.
Test serde::se::pp::test::prettyfy

After lengthy debugging, following codepath is required:

1. runtime-detection is enabled
2. x86 architecture
3. No avx2 available on system
4. No sse2 available on system
5. portable is disabled

In that code path we end up calling write_string_rust twice.
But the first one does not advance the string, so we end up writing the same string twice.
The solutions is to move the *string = &string[string.len()..]; line into write_string_rust() itself

Closes #62

[Licenser]

wonderful catch, thank you!