shachain: grammer fixes, gofmt conformance, slight interface changes

This commit contains a series of post-merge fixes which include:
  * ToBytes -> Encode
    * As the former version doesn’t _require_ allocation of new slices *
  * Store -> AddNextEntry
    * To communicate to the caller that entries MUST be added
      in order
  * Several grammar, spacing, spelling and gofmt fixes.

shachain/element_test.go

@@ -1,9 +1,10 @@
 package shachain
 
 import (
-	"github.com/go-errors/errors"
 	"reflect"
 	"testing"
+
+	"github.com/go-errors/errors"
 )
 
 // bitsToIndex is a helper function which takes 'n' last bits as input and