slice and array re-work plus some misc. changes

* `@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.

std/debug.zig

@@ -149,7 +149,7 @@ const Constant = struct {
             return error.InvalidDebugInfo;
         if (self.signed)
             return error.InvalidDebugInfo;
-        return mem.sliceAsInt(self.payload, false, u64);
+        return mem.readInt(self.payload, u64, false);
     }
 };