Non-version-specific:
* More tests

For 0.2.1:
* Multimethods + method combinations
* Missing parts of unix library (ftw, various system databases...)
* Pretty-printer
* Generalized conditions (Errors, Warnings, debugger hooks...)
* Declarations (associated to variables, not values)
* Dynamic variables (as implicit top-level environment, shadowed by
   everything else)

When needed by other code:
* Type predicates for all types
* Structures (in C code)

Sometimes:
* Documentation
* Better format
* Some platform- and locale-independent conversion of numbers from/to strings
* Unicode case-insensitive string compare
* More number types:
  - Complex numbers (FP-only)
* syntax-rules
* Sound I/O
* Compiler (into bytecode)
* JIT (using GNU Lightning)
* FFI

External modules and libraries:
* JSON

