tools: misc improvements

[slumber]

• NEXUS_CACHE environment variable for caching generated public parameters. Defaults to <crate root>/target/nexus-cache. For instance, one can do

export NEXUS_CACHE=/tmp/nexus
cargo nexus prove
cargo nexus verify

• Can save and verify sequential proofs
• Parameters are defaulted to seq with k=16. Bigger k makes parameters loading to take significant amount of time, k=64 is 2min+ on my machine. Can be configured with both -k=<k> --impl=<seq/par> or env variables

export NEXUS_VM_K=32
export NEXUS_VM_NOVAIMPL=par
cargo nexus prove
cargo nexus verify

Priority is defaults < envs < cli

• Introduce --profile option instead of --release flag, which defaults to "release". To run in debug

cargo nexus prove --profile=dev # release or any other custom profile

see https://doc.rust-lang.org/cargo/reference/profiles.html

[slumber]

@govereau see 310aef2 for clarifying the discussion #77 (comment)