Reconsider as-casting

[illicitonion]

Very early on in the course, we introduce as casting: https://rust-exercises.com/100-exercises/02_basic_calculator/10_as_casting

A few thoughts on this:

1. I'd argue in general converting between different sizes of integers is a much less core part of the language than the early placement of this section suggests - we may want to move it later (where we can also rely on more knowledge such as of traits). I don't think it's required for any of the exercises done before we could reasonably introduce Into/From and friends.
2. I think we should be default recommending Into/From and TryInto/TryFrom instead of as. In general I think as should only be used where it's strictly required, and if there's a more specific conversion mechanism we should prefer that - we shouldn't train in reaching for as as a default.
3. When we're talking about how as can cause bits to be re-interpreted, I'd recommend we throw in a link to a source on bit representation. I've had novices who only know JS/Python before (so have never encountered signed vs unsigned types) be (reasonably) confused about how to predict/interpret -1_i8 as u8 and 255_u8 as i8. We talk about how truncation keeps bytes, but without knowing what that byte representation is (i.e. what two's complement is) it can be hard to interpret. I think this can be handled with a simple one-sentence explanation and a link.