study: use checkedint as a drop-in replacement of native long

[Walter Bright]

On 8/14/2020 9:28 PM, mw wrote:
> I list 8 things I've found below. Suggestions or PR welcome, e.g. even on how to 
> improve the current implementation of `struct Checked`, or help from the compiler.

This is nice work. Thank you! Please file these issues in Bugzilla.