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.
* @enumTagName renamed to @tagName and it works on enums and
union-enums
* Remove the EnumTag type. Now there is only enum and union,
and the tag type of a union is always an enum.
* unions support specifying the tag enum type, and they support
inferring an enum tag type.
* Enums no longer support field types but they do support
setting the tag values. Likewise union-enums when inferring
an enum tag type support setting the tag values.
* It is now an error for enums and unions to have 0 fields.
* switch statements support union-enums
closes#618
see #383
there is a plan to unify most of the reflection into 2
builtin functions, as outlined in the above issue,
but this gives us needed features for now, and we can
iterate on the design in future commits
Jimmi Holst Christensen
Andrew Kelley