@indices_of

macro usz[] @indices_of(Allocator allocator, array, #predicate)

Applies a given predicate function to each element of an array and returns a new
array of `usz` values, each element representing an index within the original array
where the predicate returned `true`.

The `.len` value of the returned array can also be used to quickly identify how many
input array elements matched the predicate.

For example:
```c3
int[] arr = { 0, 20, 4, 30 };
int[] matched_indices = array::@indices_of(mem, arr, fn (u, a) => a > 10);
```

The `matched_indices` variable should contain a dynamically-allocated array of `[1, 3]`,
and thus its count indicates that 2 of the 4 elements matched the predicate condition.

Parameters