Investigate: Assess arithmetic safety of fuel-merkle crate

[xgreenx]

FuelLabs/fuel-merkle#124

[Dentosal]

Mostly addressed in #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.

[MitchTurner]

We will be creating the new implementation, so maybe we can just add this lint from the conception of the new new library.