feat: vec index trait (#92)

## Pull Request type

<!-- Please try to limit your pull request to one type; submit multiple
pull requests if needed. -->

Please check the type of change your PR introduces:

- [ ] Bugfix
- [x] Feature
- [ ] Code style update (formatting, renaming)
- [ ] Refactoring (no functional changes, no API changes)
- [ ] Build-related changes
- [ ] Documentation content changes
- [ ] Other (please describe):

## What is the current behavior?

To fetch an element at a particular index in a `Vec`, one has to use
`at`

Issue Number: N/A

## What is the new behavior?

PR implements the `Index` trait to allow for using `[]` operator when
fetching Vec elements:

```
let element_at_2 = a_vec[2];
```

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

<!-- If this does introduce a breaking change, please describe the
impact and migration path for existing applications below. -->

## Other information

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screenshots of how the component looks before and after the change. -->

alexandria/data_structures/src/tests/vec_test.cairo

@@ -1,7 +1,6 @@
 // Core lib imports
 use array::ArrayTrait;
-use traits::Into;
-use traits::TryInto;
+use traits::{Index, Into, TryInto};
 use option::OptionTrait;
 use result::ResultTrait;
 use dict::Felt252DictTrait;
@@ -88,3 +87,20 @@ fn vec_set_test_expect_error() {
     vec.set(1, 2)
 }
 
+#[test]
+#[available_gas(2000000)]
+fn vec_index_trait_test() {
+    let mut vec = VecTrait::<u128>::new();
+    vec.push(42);
+    vec.push(0x1337);
+    assert(vec[0] == 42, 'vec[0] != 42');
+    assert(vec[1] == 0x1337, 'vec[1] != 0x1337');
+}
+
+#[test]
+#[available_gas(2000000)]
+#[should_panic(expected: ('Index out of bounds', ))]
+fn vec_index_trait_out_of_bounds_test() {
+    let mut vec = VecTrait::<u128>::new();
+    vec[0];
+}

alexandria/data_structures/src/vec.cairo

@@ -15,7 +15,7 @@
 // Core lib imports
 use dict::Felt252DictTrait;
 use option::OptionTrait;
-use traits::Into;
+use traits::{Index, Into};
 
 struct Vec<T> {
     items: Felt252Dict<T>,
@@ -114,3 +114,10 @@ impl TFelt252DictValue: Felt252DictValue<T>> of VecTrait<T> {
         *(self.len)
     }
 }
+
+impl VecIndex<T, impl VecTraitImpl: VecTrait<T>> of Index<Vec<T>, usize, T> {
+    #[inline(always)]
+    fn index(ref self: Vec<T>, index: usize) -> T {
+        self.at(index)
+    }
+}