util/slicesx: add Filter function

For use in corp, where we appear to have re-implemented this in a few
places with varying signatures.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Id863a87e674f3caa87945519be8e09650e9c1d76

util/slicesx/slicesx.go

@@ -77,3 +77,17 @@ func EqualSameNil[S ~[]E, E comparable](s1, s2 S) bool {
 	}
 	return true
 }
+
+// Filter calls fn with each element of the provided src slice, and appends the
+// element to dst if fn returns true.
+//
+// dst can be nil to allocate a new slice, or set to src[:0] to filter in-place
+// without allocating.
+func Filter[S ~[]T, T any](dst, src S, fn func(T) bool) S {
+	for _, x := range src {
+		if fn(x) {
+			dst = append(dst, x)
+		}
+	}
+	return dst
+}