Cast and Overflow utilities

This utility lib helps for type casting and integer operation overflow checking. The following code block shows examples:

let a: u64 = 10;
let b: i64 = a.numeric_cast();

let a = 1.overflow_add(1);

For the first example, as conversion is not perfect for slicently lossy conversion while try_from and try_into are better. However they're too verbose in most cases, so we wrap it in the cast method and make it panic while these try_xxx methods failed.

For the second example, rust std lib provides overflow checking methods such as overflowing_add. The methods provided in this lib are one step futher, panicing when any overflow happens.

lib.rs:

Traits for datenlord.