truncating from usize -> u8 (for example) doesn't actually truncate at runtime

[DenialAdams]

Can be observed in this sample program:

https://github.com/exoticorn/microw8/blob/master/examples/rust/tunnel.rs

Since wasm doesn't give us this "for free", we'll have to add a runtime implementation ourself and call when appropriate.

As a workaround, forcing a store (by going to an intermediate local) should resolve the issue

[DenialAdams]

overflow has the same issue. bleh. we could just insert a store followed by a load to force the wrapping?

[DenialAdams]

Created #45 for overflow

[DenialAdams]

Closed in d8300c8