Rust issues

[dhardy]

This tracks work-arounds which are waiting for Rust language or standard libary
changes.

• std::io::Read::read_exact() — is unstable
• How to copy data from one fixed-size array to another?
• How to simultaneously XOR all entries?
• Use SIMD library?
• Use NonZero optimisation?
• Associated constants (see Element serialisation #25) and BYTES in sum.rs.
• Error handling: when new try blocks or similar are available some code could
  be improved.
• Constant expressions: want max in sum.rs
• Borrow of item in if-let prevents usage in else part
• Trait up-casts issue 5665
• box value notation (Rust issue)
• Code tag #0018

[dhardy]

SIMD stuff moved to #31

[dhardy]

• step_by in error.rs: impl<'a> fmt::Display for ReadErrorFormatter<'a>