Add trussed-core crate

[robin-nitrokey]

• move relevant modules into trussed-core crate
  • #178
• review public dependencies
  • serde-indexed
    • #183
  • heapless
    • #186
• review public API
  • exhaustive enums
    • #189
  • non-sealed traits
  • module structure / re-exports
    • #190
  • Id::hex etc.
    • #186
  • api::NotBefore → types::NotBefore
    • #188
• add feature flags
  • clients
    • #185
  • mechanisms
    • #159
• Update docs.rs config
  • #191
• release