* `@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.
