Implement 'Collection.first_position(where:)'

StandardLibrary/Sources/Core/Collection.hylo

@@ -37,6 +37,19 @@ public extension Collection {
     return r
   }
 
+  /// Returns the position of the first element of `self` satisfying `predicate`, or
+  /// `end_position()` if no such element exists.
+  ///
+  /// - Complexity: O(n), where n is the number of elements in `self`.
+  public fun first_position<E>(where predicate: [E](Element) -> Bool) -> Position {
+    var i = start_position()
+    let j = end_position()
+    while (i != j) && !predicate(self[i]) {
+      &i = self.position(after: i)
+    }
+    return i
+  }
+
   /// Returns the result of applying `combine` on an accumulator, initialized with `initial_value`,
   /// and each element of `self`, in order.
   ///

Tests/LibraryTests/TestCases/CollectionTests.hylo

@@ -18,6 +18,11 @@ fun test_count() {
   precondition(a.count() == 64)
 }
 
+fun test_first_position() {
+  let a = 4
+  precondition(a.first_position(where: fun (_ x) { x.copy() }) == 2)
+}
+
 fun test_reduce() {
   var a = Array<Int>()
   &a.append(1)
@@ -50,6 +55,7 @@ fun test_all_satisfy() {
 
 public fun main() {
   test_count()
+  test_first_position()
   test_reduce()
   test_contains_where()
   test_all_satisfy()