u48::From<u64> should not be implemented, and so on; missing truncation function (like `as`)

The primitive unsigned integer types only implement Smaller::from(bigger) and Bigger::try_from(smaller). They do not implement Bigger::from(smaller), since that could cause truncation. Instead, as is used to convert without overflow checks and with truncation.

These Uint types should work the same way, to avoid accidentally truncating values. Instead, there should be the following options:

from(...) - only if the conversion is guaranteed to work
try_from(...).unwrap() - similar behavior to new() currently, panic on overflow
from_truncated() - or something like that, for an analogous version of primitive as

(Furthermore, it does not seem to be documented whether the Uint::From implementation truncates or panics on overflow.)

danlehmann / arbitrary-int

u48::From<u64> should not be implemented, and so on; missing truncation function (like `as`) #37