Remove the obsoleted immediate pragma

https://nim-lang.github.io/Nim/manual#templates-typed-vs-untyped-parameters

Updated the minimum required Nim version to the latest stable as of
today; v0.20.0.

> A template where every parameter is untyped is called an immediate
  template. For historical reasons templates can be explicitly
  annotated with an immediate pragma and then these templates do not
  take part in overloading resolution and the parameters' types are
  ignored by the compiler. Explicit immediate templates are now
  deprecated.

Remove deprecated module unsigned

https://github.com/nim-lang/Nim/pull/10215/files#diff-4f28823dcd8d520365b20d7f71b0d28c

Fix for not operator now being strictly unary

Backward compatible change in nim devel