Casting a felt to a felt pointer results in Int instead of Relocatable

[notlesh]

Describe the bug

When casting a felt to a felt pointer in Cairo0 (such as cast(0, felt*)), cairo-vm writes an Integer to memory instead of a Relocatable.

The OS code does this in a number of cases like: this. This particular case is later used with add_relocation_rule() which always fails because it will only accept a Relocatable.

The following example Cairo code demonstrates the problem:

from starkware.cairo.common.alloc import alloc
func main() {
    alloc_locals;

    let ptr_from_alloc: felt* = alloc();
    let ptr_from_cast_0: felt* = cast(0, felt*);

    %{ breakpoint() %}

    ret;
}

This results in:

[0] "ptr_from_alloc": "RelocatableValue(Relocatable { segment_index: 4, offset: 0 })"
[1] "ptr_from_cast_0": "Int(0)"

To Reproduce

Reproducing this issue can be done by using the cairo0_run binary in this PR. Something like this:

1. Save the cairo snippet above as test_cast.cairo.
2. Compile it with cairo-compile, e.g. cairo-compile test_cast.cairo --output test_cast_compiled.json
3. Run the SNOS cairo0_run binary with the output, e.g. cargo run -p cairo0_run -- --input test_cast_compiled.json
4. The breakpoint hint will print ids_data, showing the values of the variables

Expected behavior

Based on the OS code in cairo-lang I believe the expectation is that cast in this case should return a Relocatable, but I'm not entirely clear on this.

What version/commit are you on?

58363ad8065ed891e3b14a8191b707677c7c7cb5b9d10030822506786d8d8108 (roughly v1.0.1)

Additional context

We have created a workaround in SNOS here.

[notlesh]

Closing this issue as the problem is more intricate (described in #1860). To be clear: the problem described here seems to be "works as intended."

[pefontana]

great! @notlesh !
let's continue in #1860