fix: substraction between relocatables should be signed (#1218)

* fix: substraction between relocatables should be signed

It is valid as per the reference implementation to substract two
relocatables `a - b` where `b.offset > a.offset`.
The fix consists in not using the trait as implemented for `Relocatable`
(where it represents a positive distance between pointers) but to
promote the offsets to `i128` and only substract them for
`MaybeRelocatable`, as those are the language-level types.

* chore: remove uneeded stuff

* Add fix holes to if_reloc_equal.cairo test

* chore: changelog

---------

Co-authored-by: Pedro Fontana <fontana.pedro93@gmail.com>

CHANGELOG.md

@@ -6,6 +6,8 @@
 
 #### [0.5.1] - 2023-6-7
 
+* fix: substraction of `MaybeRelocatable` always behaves as signed [#1218](https://github.com/lambdaclass/cairo-rs/pull/1218)
+
 * fix: fix overflow for `QUAD_BIT` and `DI_BIT` hints [#1209](https://github.com/lambdaclass/cairo-rs/pull/1209)
   Fixes [#1205](https://github.com/lambdaclass/cairo-rs/issue/1205)
 

cairo_programs/if_reloc_equal.cairo

@@ -0,0 +1,16 @@
+from starkware.cairo.common.alloc import alloc
+
+func main{}() {
+    let arr: felt* = alloc();
+
+    assert arr[0] = 1;
+    assert arr[5] = 2;
+
+    let end = arr + 5;
+
+    if (arr == end) {
+        ret;
+    }
+
+    ret;
+}

src/tests/cairo_run_test.rs

@@ -967,3 +967,9 @@ fn cairo_run_reduce() {
     let program_data = include_bytes!("../../cairo_programs/reduce.json");
     run_program_simple(program_data.as_slice());
 }
+
+#[test]
+fn cairo_run_if_reloc_equal() {
+    let program_data = include_bytes!("../../cairo_programs/if_reloc_equal.json");
+    run_program_simple_with_memory_holes(program_data, 4);
+}

src/types/relocatable.rs

@@ -256,7 +256,9 @@ impl MaybeRelocatable {
                 MaybeRelocatable::RelocatableValue(rel_b),
             ) => {
                 if rel_a.segment_index == rel_b.segment_index {
-                    return Ok(MaybeRelocatable::from(Felt252::new((*rel_a - *rel_b)?)));
+                    return Ok(MaybeRelocatable::from(Felt252::from(
+                        rel_a.offset as i128 - rel_b.offset as i128,
+                    )));
                 }
                 Err(MathError::RelocatableSubDiffIndex(Box::new((
                     *rel_a, *rel_b,