* test: move tests for rln, multiplier and private-settlements to tests folder
* test: fix rln test for poseiden tree
* test: move tests for rln-wasn to standalone tests folder
* test: move tests for semaphore and utils to standalone tests folder
* chore: cargo fmt
* test: rename test files
* test: move tests out from rln/src/public to test folder
* test: fix rln tests, move tests accessign private fields back to relevant source file
* chor: run cargo fmt
* test: move all semaphore tests to tests folder
* chore: cargo fmt
* fix(rln): throw if attempting to insert out of bounds
* chore(rln): better error msg
* fix(merkle-tree): make it mimic OptimalMerkleTree impl
* chore(rln): return result as is