Memory allocation pitfalls

There are a couple of common pitfalls that unnecessary allocates memory and may seriously impact performance by:

• introducing GC latency
• increasing memory fragmentation
• introducing unnecessary indirection
• introducing unnecesary instruction
• preventing compiler optimizations

Example: https://github.com/status-im/nim-beacon-chain/pull/1573

Those common ways are:

• [] slicing instead of toOpenArray
• Iterating with implicit items on an array/array fields. This is solved in Nim devel (https://github.com/nim-lang/Nim/issues/14421), but not in the 1.2.x branch that we target.