Add operation tracking to VmStateIterator

[bobbinth]

Currently, the VmState struct which is returned from the VmStateIterator does not contain the operation which was executed to put the VM into this state. We should add another field to this struct so that it looks something like this:

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct VmState {
    pub op: Operation,
    pub clk: usize,
    pub fmp: Felt,
    pub stack: Vec<Felt>,
    pub memory: Vec<(u64, Word)>,
}

The op field would need to be populated from the information in the decoder. The decoder doesn't explicitly track the operations yet. It is possible to infer the operations from the trace, but I think a better approach would be to have a vector of operations in the decoder struct. Then, as the VM executes operations, they would be pushed into this vector.

Since tracking operations would result in some overhead, the above should happen only when we are executing programs via execute_iter() function.

Implementing this functionality will enable counting operations executed by the VM which would be useful for things like #198.

[tohrnii]

@bobbinth I can complete this first as one PR and use that to complete #198.

[bobbinth]

That would be awesome! Thank you!

[bobbinth]

Closed by #251