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.
* docgen supports obj_err code kind for demonstrating
errors without explicit test cases
* add documentation for `extern enum`. See #367
* remove coldcc keyword and add @setIsCold. See #661
* add compile errors for non-extern struct, enum, unions
in function signatures
* add .h file generation for extern struct, enum, unions
