*: Add Digest struct to get bytes of digest

[crazycs520]

What problem does this PR solve?

As the title said.

What is changed and how it works?

Check List

Tests

• Unit test

Code changes

• Has exported function/method change
• Has exported variable/fields change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• N/A

[tiancaiamao]

/LGTM

[lysu]

/lgtm