Add new type U128

[nfurfaro]

We need a type to contain the result of multiplying 2 u64s for example.
This should be opt-in, as in the default when performing math ops with a type should be to return the same type.

Bikeshedding discussion here: https://fuellabs.slack.com/archives/C01KBNGSWGJ/p1651596324523349

[simonr0204]

Started working on this here https://github.com/FuelLabs/sway/blob/simon-u128/sway-lib-std/src/u128.sw

Should methods on the U128 type be over and underflow safe by default? Could be a lot of overhead there if it means returning a Result from every arithmetic operation

[adlerjohn]

Should methods on the U128 type be over and underflow safe by default?

Start with the minimum (panic on overflow), then add more functionality as we need.