Implement a language for writing langauge definitions

[zerbina]

This issue tracks progress on the meta-language and its associated tooling.

• interpreter: represent all numbers as rationals #119
• implement turning a LangDef into a semantically self-contained textual formal definition
• add support for extending LangDefs (removing/adding/altering types and rules)
• add support for writing transformations (in a declarative manner)
• add support for defining parsers (unicode character string -> AST)
• replace the passtool with meta-language tooling
• decouple semantic analysis from NimNode input AST
• add a command-line shell for LangDef queries #121