Fix semaphore v4

[vivianjeng]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (yarn build) was run locally in root directory and any changes were pushed
• Lint (yarn lint:fix) was run locally in root directory and any changes were pushed

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: close #541

What is the new behavior?

• Semaphore v4

1. Variable names:
   1. External Nullifier → Scope
   2. Siblings → Tree Siblings
   3. Path Indices → Tree Indices
2. Simplified identity: The two old identity secrets (trapdoor and nullifier) have been replaced by 1 secret only (secret). So identity commitment = hash(identity secret), and nullifier hash = hash(scope, identity secret).

Does this introduce a breaking change?

• Yes
• No

Other information