refactor the Nushell modules

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