Design unit system

[narrowtux]

TODO

• Registry for unit strings to their definitions
  • Possibility to use aliases (for example meter to m)
• Prepare lexer and parser to parse unit expressions
  • m / s should be parsed as {:divide, {:unit, "m"}, {:unit, "s"}}
• Add second parser-pass to map {:unit, binary} to a unit definition
• Change eval/2 functions to support multiplication operations (/, *, ^) on amounts
• Support SI prefixes (m, k, M, ...)
• function to convert units