Re-organize STM library structure

[jpraynaud]

Why

We want to have more readability of the STM library and make it easier to maintain.

What

Reorganize the code of the STM library:

• Long files that we want to split
• Structure definition is too far from to the implementations blocks
• Modules split and re-organization

How

• Structure is not close to the implementations blocks
• Refactor modules into:
  • aggregate_signature
  • bls_multi_signature
  • eligibility_check
  • errors
  • individual_signature
  • key_registration
  • merkle_tree
  • parameters
  • participant
• Remove the deprecated batch-verify-aggregates feature
• Rename the SigningKey::gen to SigningKey::generate to prepare switch to Rust 2024

[curiecrypt]

@jpraynaud We should also have a look at the:

• tests/
• examples/
• benches/