Harnesses for to_bytes and to_bytes_with_nul

[Yenyun035]

Towards #150

Changes

• Added harnesses for to_bytes and to_bytes_with_nul
• Added a small fix to count_bytes

Verification Result:

Checking harness ffi::c_str::verify::check_to_bytes_with_nul...

VERIFICATION RESULT:
 ** 0 of 179 failed (5 unreachable)

VERIFICATION:- SUCCESSFUL
Verification Time: 88.397385s

Checking harness ffi::c_str::verify::check_to_bytes...

VERIFICATION RESULT:
 ** 0 of 180 failed (5 unreachable)

VERIFICATION:- SUCCESSFUL
Verification Time: 79.66312s

Checking harness ffi::c_str::verify::check_from_bytes_until_nul...

VERIFICATION RESULT:
 ** 0 of 132 failed (5 unreachable)

VERIFICATION:- SUCCESSFUL
Verification Time: 28.593569s

Complete - 3 successfully verified harnesses, 0 failures, 3 total.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[celinval]

Thanks!