Add back forwarding func to deduplicate doc comments

As of Go 1.13 this can be inlined so it doesn't add overhead.

Fixes #22.

xxhash.go

diff --git a/xxhash.go b/xxhash.go
index db0b35f..8f34f5d 100644
--- a/xxhash.go
+++ b/xxhash.go
@@ -8,6 +8,9 @@
 	"math/bits"
 )
 
+// Sum64 computes the 64-bit xxHash digest of b.
+func Sum64(b []byte) uint64 { return sum64(b) }
+
 const (
 	prime1 uint64 = 11400714785074694791
 	prime2 uint64 = 14029467366897019727

xxhash_amd64.go

diff --git a/xxhash_amd64.go b/xxhash_amd64.go
index ad14b80..16df852 100644
--- a/xxhash_amd64.go
+++ b/xxhash_amd64.go
@@ -4,10 +4,8 @@
 
 package xxhash
 
-// Sum64 computes the 64-bit xxHash digest of b.
-//
 //go:noescape
-func Sum64(b []byte) uint64
+func sum64(b []byte) uint64
 
 //go:noescape
 func writeBlocks(d *Digest, b []byte) int

xxhash_amd64.s

diff --git a/xxhash_amd64.s b/xxhash_amd64.s
index d580e32..789316d 100644
--- a/xxhash_amd64.s
+++ b/xxhash_amd64.s
@@ -38,8 +38,8 @@
 	IMULQ R13, acc \
 	ADDQ  R15, acc
 
-// func Sum64(b []byte) uint64
-TEXT ·Sum64(SB), NOSPLIT, $0-32
+// func sum64(b []byte) uint64
+TEXT ·sum64(SB), NOSPLIT, $0-32
 	// Load fixed primes.
 	MOVQ ·prime1v(SB), R13
 	MOVQ ·prime2v(SB), R14

xxhash_other.go

diff --git a/xxhash_other.go b/xxhash_other.go
index 4a5a821..be7cea2 100644
--- a/xxhash_other.go
+++ b/xxhash_other.go
@@ -2,8 +2,7 @@
 
 package xxhash
 
-// Sum64 computes the 64-bit xxHash digest of b.
-func Sum64(b []byte) uint64 {
+func sum64(b []byte) uint64 {
 	// A simpler version would be
 	//   d := New()
 	//   d.Write(b)