[gsoc] dis interpreter

[Alexandre Esteves]

Hey

Last year's code had very leaky abstractions when translating memory<->objects, and gradually built up technical debt.

I rewrote much of the code from scratch, and now the memory read/write related aspects are pretty well isolated.

Everything else now is normal Dart code. Reference counting is also handled automatically, except in 2-3 places (temporarily).

Threads, channels and loading of modules (built-in and libraries) was added.

From the Sys module, functions that do not use file descriptors have been implemented.

I'll try to get the other ones working before I go on vacation next week.

As always, the repository is https://bitbucket.org/alexandre_esteves/disondart

This year's efforts are in /web/bin/rewrite, and the interesting files are rewrite/instructions/isa.dart & rewrite/types/semantics.dart.