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.
* add `setFnTest`, `setFnVisible`, `setFnStaticEval`,
`setFnNoInline` builtin functions to replace previous
directive functionality
* add `coldcc` and `nakedcc` as keywords which can be used as part
of a function prototype.
* `setDebugSafety` builtin can be used to set debug safety features
at a per block scope level.
* closes#169
Andrew Kelley