* Introduce the concept of packages. Closes#3
* Add support for error notes.
* Introduce `@import` and `@c_import` builtin functions and
remove the `import` and `c_import` top level declarations.
* Introduce the `use` top level declaration.
* Add `--check-unused` parameter to perform semantic
analysis and codegen on all top level declarations, not
just exported ones and ones referenced by exported ones.
* Delete the root export node and add `--library` argument.
Andrew Kelley