Enhance STM library readability - 2/2

[jpraynaud]

Why

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

What

Use better struct fields namings and revise documentation.

How

• Remove existing TODOs (and prepare list of those which need to be addressed separately)
• Define new names for struct fields:
  • Assess all the modified public fields (that will not be backward compatible, and eventually bump the minor version)
  • Create golden tests for public structs with renamed fields
  • Rename the fields
• Split the tests in aggregate_signature/mod.rs and move them in a test sub-module
• Enhance the documentation
• Update protocol parameters in benchmark (and in documentation) to make them more realistic
• Run a backward compatibility check