The purpose of this is:
* Only one way to do things
* Changing a function with void return type to return a possible
error becomes a 1 character change, subtly encouraging
people to use errors.
See #632
Here are some imperfect sed commands for performing this update:
remove arrow:
```
sed -i 's/\(\bfn\b.*\)-> /\1/g' $(find . -name "*.zig")
```
add void:
```
sed -i 's/\(\bfn\b.*\))\s*{/\1) void {/g' $(find ../ -name "*.zig")
```
Some cleanup may be necessary, but this should do the bulk of the work.
* 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.
