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.
* `@truncate` builtin allows casting to the same size integer.
It also performs two's complement casting between signed and
unsigned integers.
* The idiomatic way to convert between bytes and numbers is now
`mem.readInt` and `mem.writeInt` instead of an unsafe cast.
It works at compile time, is safer, and looks cleaner.
* Implicitly casting an array to a slice is allowed only if the
slice is const.
* Constant pointer values know if their memory is from a compile-
time constant value or a compile-time variable.
* Cast from [N]u8 to []T no longer allowed, but [N]u8 to []const T
still allowed.
* Fix inability to pass a mutable pointer to comptime variable at
compile-time to a function and have the function modify the
memory pointed to by the pointer.
* Add the `comptime T: type` parameter back to mem.eql. Prevents
accidentally creating instantiations for arrays.
Andrew Kelley
Marc Tiehuis
Jimmi Holst Christensen