Avoid `Felt` in the SDK API where finite field arithmetic is unwanted

[greenhat]

Following the discouraging of using Felt to avoid finite field arithmetics in the docs repo at 0xMiden/docs#177, and the discussions elsewhere.

Why

Finite field arithmetic differs from integer arithmetic (wrapping, division). The asset quantity manipulation is one area where it can lead to unwanted results.

How

Use integer types in the SDK API where we want to avoid the finite field arithmetics. E.g. make the Asset::amount(&self) -> u64 and an Asset constructor for the fungible asset to accept the amount as u64 with bounds checking.

@bobbinth what do you think?

/cc @Keinberger

[greenhat]

We can also remove the Div trait implementation for the Felt type to break the / operation.

EDIT: And introduce it as the felt_inversion(a, b) function. Although this would hurt the felt ops discoverability.