refactor - performance

[clach04]

Use a profiler to find hotspots. There are inefficient pieces of code, for example

• single slices, rather than multiple
• (Byte) concatenation, compare:
  • demos concat faster - https://blog.ganssle.io/articles/2019/11/string-concat.html#benchmark-code
  • demos join faster - https://discuss.python.org/t/joining-strings-is-faster-than-adding-but-only-if-concatenating-3-strings/5293
• struct/array calls
• big ticket item likely to be key derivation called all the time for read and write encrypted_file() (not an issue for the class interface) - #11
• use a package that supports CBC, with manually applying it. Blowfish.new(key, mode=Blowfish.MODE_ECB). See #3