Nim fusion

Starting from this version you can run nim fusion to install or update the fusion package - an extension to the nim stdlib with some additional modules.

Currently, the following modules are present:

• fusion/smartptrs - C++-like unique/shared pointers
• fusion/btreetables - sorted associative containers
• fusion/matching - pattern matching implementation using nim macros
  • introduction article: https://nim-lang.org/blog/2021/01/11/pattern-matching.html
• fusion/htmlparser - HTML parser
• fusion/astdsl - karax-style DSL for constructing an AST
• fusion/filepermissions - convenience functions for working with file permissions.

Documentation index can be found here, project repository: link.