Mojo change: renaming `alias` to `comptime`

All right, just to give everyone a heads up. After quite a bit of debate, we’re moving forward with implementing ‘comptime’ in Mojo.

In today’s nightly, and in the 25.7 release (next week) Mojo compiler will take both alias and comptime as synonyms. Error messages, examples and documentation will be gradually converted to comptime. In the long run, we’d like to make alias deprecated, but that will take months and would be a multi-month transition.

Thank you everyone for your insights!