Major update (towards first milestone)

- Added a lot of new natives (also, eval and load are now natives
instead of syntax)
- Added let syntax (same semantics as in Slip)
- Switched from pair- to array-based storage
- Added a real ‘REPL’  (and a simple printer for prettier output in
REPL)
- Added I/O module to implement natives like read, load, …
- Added support for dot syntax in parser

EXTRA: the original ’mceval.slip’ can now be run on top of this
interpreter if you omit evaluate-load (because open-input-file etc.
still need to be implemented)