Add .Filter for iterator method chaining

Author: William-Young-97

README.md

@@ -5,9 +5,9 @@
 A general purpose library offering functional helpers for Golang.
 
 ```go
-// Find the first 5 prime numbers
-primes := iter.Filter(iter.Count(), isPrime).Take(5).Collect()
-reflect.DeepEqual(t, primes, []int{2, 3, 5, 7, 11})
+// Find the first 5 even numbers
+evens := iter.Count().Filter(filters.IsEven).Take(5).Collect()
+reflect.DeepEqual(evens, []int{0, 2, 4, 6, 8})
 ```
 
 _[Read the docs.](https://pkg.go.dev/github.com/BooleanCat/go-functional)_
@@ -51,9 +51,8 @@ Here are a few trivial example of what's possible using the iterators in this
 library.
 
 ```go
-// All even natural numbers (2, 4, 6, 8...)
-isEven := func(n int) bool { return n%2 == 0 }
-evens := iter.Filter(iter.Count().Drop(1), isEven)
+// All odd natural numbers (1, 3, 5, 7...)
+odds := iter.Count().Drop(1).Filter(filters.IsOdd).Collect()
 ```
 
 ```go

iter/iter.go

@@ -121,3 +121,9 @@ func (iter *BaseIter[T]) Drop(n uint) *DropIter[T] {
 func (iter *BaseIter[T]) Take(n uint) *TakeIter[T] {
 	return Take[T](iter, n)
 }
+
+// Filter is a convenience method for [Filter], providing this iterator
+// as an argument.
+func (iter *BaseIter[T]) Filter(fun func(T) bool) *FilterIter[T] {
+	return Filter[T](iter, fun)
+}

iter/iter_test.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/BooleanCat/go-functional/internal/assert"
 	"github.com/BooleanCat/go-functional/iter"
+	"github.com/BooleanCat/go-functional/iter/filters"
 	"github.com/BooleanCat/go-functional/iter/ops"
 	"github.com/BooleanCat/go-functional/option"
 )
@@ -179,3 +180,8 @@ func TestBaseIteratorTake(t *testing.T) {
 	items := iter.Lift([]int{1, 2, 3}).Take(2).Collect()
 	assert.SliceEqual(t, items, []int{1, 2})
 }
+
+func TestBaseIteratorFilter(t *testing.T) {
+	items := iter.Lift([]int{1, 2, 3}).Filter(filters.IsEven[int]).Collect()
+	assert.SliceEqual(t, items, []int{2})
+}