Add a way to iterate arrays with index

### What is wrong?

Currently, the only way to iterate arrays with a for loop goes like this:

```
for val in my_array:
pass # do something with value
```

It's common that one also needs to obtain the index of the value which currently can only be obtained manually e.g.

```
let index: u256 = 0
for val in my_array:
pass # do something with value
index += 1
```

It would be handy to have something like `enumerate()` to get both index and value at the same time e.g.

```
for (index, val) in my_array.enumerate():
pass # do something with index and value
```

### How can it be fixed

Discuss & :robot:

Start by locating the array for-loop implementation and the entry points for array methods in the compiler. Clarify the proposed enumerate() syntax and index/value semantics, then define completion around supporting the issue's example and adding coverage for indexed iteration.

Written by the indexing model from the issue text.