refactor the Nushell modules (!158)

this MR is two-fold
- it restructures the two main Nushell modules so that they are easier to read and use
- it improves the "run" and "plot" modules for the benchmarks

## changelog
- `.nushell/` is now renamed to `nu-utils/`
- `benchmarks/` is now a valid Nushell module which exports a bunch of modules
  - `benchmarks linalg`: measure and plot linear algebra operations
  - `benchmarks setup`: measure and plot trusted setup building
  - `benchmarks commit`: measure and plot crafting commitments
  - `benchmarks recoding`: measure and plot the recoding of shards
  - `benchmarks fec`: measure and plot FEC operations, such as encoding and recoding, and allow combining these results with the pure recoding ones
- the submodules of `benchmarks` typically have a `run` and a `plot` command, whith the exception of `benchmarks fec` which has a `run` module and multiple "plot" commands in `benchmarks fec plot`
- the "run" commands will create a random temp file by default and ask for confirmation otherwise if the output file already exists, unless `--force` is used
- snippetds in `benchmarks/README.md` have been updated