Reduce TCB, compiled artifact size, compilation speed

This documents ways to reduce the TCB and compiled artifact size, and tracks a few leads.

Dependencies

• Use fewer
• Avoid duplicates in the build tree (see Cargo.lock, or cargo tree)
• Replace dependencies with similar ones that are already in our build tree
  • replace xxhash with ahash (see #1074)
  • memsec with zeroize (+ sth for the constant time comparisons)
  • replace sha2 with provided implementation (see pgp-cert-d#52)
• make features (and dependencies) optional

Code size

• Reduce opportunities for monomorphization:
  • Don't make arguments compile-time polymorphic
  • Exception: small public functions for ergonomics, implementation should be an internal function which only uses concrete types
• Reduce code duplication
• Reduce code duplication caused by the use of macros

Useful tools

• cargo tree
• cargo bloat