Convert `as` conversions to `From`/`TryFrom`

[crepererum]

The as conversions in our protocol code our quite dangerous and I think we might have a few integer overflow bugs in there. In general I think we should replace all as conversions with From and TryFrom to make it clear if the conversion can fail or not. There is also a clippy lint as_conversions which should be added to our code base to prevent regressions.

[pierwill]

👋 I'd like to work on this.