syntax: functions require return type. remove ->

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.

test/assemble_and_link.zig

@@ -1,7 +1,7 @@
 const builtin = @import("builtin");
 const tests = @import("tests.zig");
 
-pub fn addCases(cases: &tests.CompareOutputContext) {
+pub fn addCases(cases: &tests.CompareOutputContext) void {
     if (builtin.os == builtin.Os.linux and builtin.arch == builtin.Arch.x86_64) {
         cases.addAsm("hello world linux x86_64",
             \\.text