add skeleton for shachain packaged

* Still considering if it’s better just to use a modified HD chain or
not:
  * give them child branch/sub-tree etc.

wallet/shachain/hash_chain.go

@@ -0,0 +1,32 @@
+package wallet
+
+import "github.com/btcsuite/btcd/wire"
+
+// TODO(roasbeef): port Rusty's hash-chain stuff
+//  * or just use HD chains based off of CodeShark's proposal?
+
+// chainFragment...
+type chainFragment struct {
+	index uint64
+	hash  wire.ShaHash
+}
+
+// ShaChain...
+type ShaChain struct {
+	lastChainIndex uint64
+
+	chainFragments []chainFragment
+}
+
+// NewShaChain...
+func NewShaChain(seed wire.ShaHash) *ShaChain {
+	// TODO(roasbeef): from/to or static size?
+}
+
+// NextHash...
+func (s *ShaChain) NextHash() {
+}
+
+// GetHash...
+func (s *ShaChain) GetHash(index uint64) {
+}

wallet/shachain/hash_chain_test.go

@@ -0,0 +1 @@
+package wallet