Refactored client's symmetric_decrypt_and_verify()

[BogdanYarotsky]

Hello, I'm trying to fix the issue that is mentioned in #207 and would like to clean up symmetric_decrypt_and_verify() first.

Here is a list of small changes that I made:

1. Removed commented out old code
2. Simplified indexing of slices (using slice directly instead of dst[..], removed "all" variable")
3. Unified trace message + signature verifying in 2 match arms (Sign, SignAndEncrypt) into a new method

Please let me know if any of the removed code is still needed or I missed something during the refactoring.

Thanks!

[BogdanYarotsky]

Changes are now split in few PRs for easier review:
#379 #380 #381