Investigate: Assess arithmetic safety of fuel-merkle crate

[xgreenx]

https://github.com/FuelLabs/fuel-merkle/issues/124

[Dentosal]

Mostly addressed in https://github.com/FuelLabs/fuel-vm/pull/729

From the original issue there's still:

• Limit leaves_count to 2^63 for push and load operations

Which seems like a non-issue, as a it's physically impossible to have enough memory to for trees of that magnitude.