Refactor backends

[zlonast]

docs:
- plonk
- rollup
- symbolic

scripts:
- local-testnet
- plonk
- rollup
- symbolic

zkfold-backends:
- plonk
- rollup
- symbolic
- symbolic-balancing
- zkfold-backends.cabal

zkfold-example:
- src 
-- equalityCheckContract
-- identityCircuit
- zkfold-example.cabal

zkfold-contracts:
- becnh
-- bench-cpu-mem
-- bench-rollup
- src
-- PlonkVerifierToken
-- PlonkVerifierTx
-- Rollup
-- ...
- zkfold-contracts.cabal

zkfold-cardano:
- src
- test
- zkfold-cardano.cabal

[zlonast]

In the next pull requests I will write more tests, as it is quite difficult to check that everything works.

[zlonast]

@vlasin so far I've tested everything manually