types/views: add LenIter method to slice view types

This is basically https://github.com/bradfitz/iter which was
a joke but now that Go's adding range over int soonish, might
as well. It simplies our code elsewher that uses slice views.

Updates #8948

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/views/views.go

@@ -73,6 +73,11 @@ func (v SliceView[T, V]) IsNil() bool { return v.ж == nil }
 // Len returns the length of the slice.
 func (v SliceView[T, V]) Len() int { return len(v.ж) }
 
+// LenIter returns a slice the same length as the v.Len().
+// The caller can then range over it to get the valid indexes.
+// It does not allocate.
+func (v SliceView[T, V]) LenIter() []struct{} { return make([]struct{}, len(v.ж)) }
+
 // At returns a View of the element at index `i` of the slice.
 func (v SliceView[T, V]) At(i int) V { return v.ж[i].View() }
 
@@ -129,6 +134,11 @@ func (v Slice[T]) IsNil() bool { return v.ж == nil }
 // Len returns the length of the slice.
 func (v Slice[T]) Len() int { return len(v.ж) }
 
+// LenIter returns a slice the same length as the v.Len().
+// The caller can then range over it to get the valid indexes.
+// It does not allocate.
+func (v Slice[T]) LenIter() []struct{} { return make([]struct{}, len(v.ж)) }
+
 // At returns the element at index `i` of the slice.
 func (v Slice[T]) At(i int) T { return v.ж[i] }
 
@@ -233,6 +243,11 @@ func (v IPPrefixSlice) IsNil() bool { return v.ж.IsNil() }
 // Len returns the length of the slice.
 func (v IPPrefixSlice) Len() int { return v.ж.Len() }
 
+// LenIter returns a slice the same length as the v.Len().
+// The caller can then range over it to get the valid indexes.
+// It does not allocate.
+func (v IPPrefixSlice) LenIter() []struct{} { return make([]struct{}, v.ж.Len()) }
+
 // At returns the IPPrefix at index `i` of the slice.
 func (v IPPrefixSlice) At(i int) netip.Prefix { return v.ж.At(i) }