Investigate the performance regression in onchain decider

[winderica]

The onchain decider SNARK without the light-test feature now takes longer time and more RAM. In fact, the setup and proof generation could be done with 32GB of RAM in 2 minutes, as reported in #88. However, the onchain decider now takes more than 3 minutes and eventually fails to run on my machine with 64GB of RAM:

> cargo test --package folding-schemes --lib --release -- folding::nova::decider_eth::tests::test_decider --exact --nocapture
    Finished `release` profile [optimized] target(s) in 0.19s
     Running unittests src/lib.rs (target/release/deps/folding_schemes-850178000889a0b5)

running 1 test
Start:   KZG10::Setup with degree 65536
··Start:   Generating powers of G
····End:     Generating powers of G ................................................87.030ms
··Start:   Generating powers of gamma * G
····End:     Generating powers of gamma * G ........................................91.847ms
··Start:   Generating negative powers of h in G2
····End:     Generating negative powers of h in G2 .................................296ns
··End:     KZG10::Setup with degree 65536 ..........................................182.324ms
Start:   Groth16::Generator
··Start:   Constraint synthesis
····End:     Constraint synthesis ..................................................17.176s
··Start:   Inlining LCs
test folding::nova::decider_eth::tests::test_decider has been running for over 60 seconds
error: test failed, to rerun pass `-p folding-schemes --lib`

Caused by:
  process didn't exit successfully: `/home/winderica/ivc/folding-schemes/target/release/deps/folding_schemes-850178000889a0b5 'folding::nova::decider_eth::tests::test_decider' --exact --nocapture` (signal: 9, SIGKILL: kill)