Add hilbert - Mapping values to and from space-filling curves. (#1075)

Such as Hilbert and Peano curves.

README.md

@@ -217,6 +217,7 @@ Please take a quick gander at the [contribution guidelines](https://github.com/a
 * [gods](https://github.com/emirpasic/gods) - Go Data Structures. Containers, Sets, Lists, Stacks, Maps, BidiMaps, Trees, HashSet etc.
 * [golang-set](https://github.com/deckarep/golang-set) - Thread-Safe and Non-Thread-Safe high-performance sets for Go.
 * [goskiplist](https://github.com/ryszard/goskiplist) - A skip list implementation in Go.
+* [hilbert](https://github.com/google/hilbert) - Go package for mapping values to and from space-filling curves, such as Hilbert and Peano curves.
 * [levenshtein](https://github.com/agnivade/levenshtein) - Implementation to calculate levenshtein distance in Go.
 * [mafsa](https://github.com/smartystreets/mafsa) - MA-FSA implementation with Minimal Perfect Hashing
 * [roaring](https://github.com/RoaringBitmap/roaring) - Go package implementing compressed bitsets.