aezeed: add a set of benchmarks

In this commit we add a set of benchmarks to be able to measure the
enciphering and deciphering speed of the current scheme with the
current scrypt parameters.

On my laptop I get about 100ms per attempt:
⛰ go test -run=XXX -bench=.

goos: darwin
goarch: amd64
pkg: github.com/lightningnetwork/lnd/aezeed
BenchmarkToMnenonic-4     	      10	 102287840 ns/op
BenchmarkFromMnenonic-4   	      10	 105874973 ns/op
PASS
ok  	github.com/lightningnetwork/lnd/aezeed	3.036s

aezeed/bench_test.go

@@ -0,0 +1,69 @@
+package aezeed
+
+import (
+	"testing"
+	"time"
+)
+
+var (
+	mnemonic Mnemonic
+
+	seed *CipherSeed
+)
+
+// BenchmarkFrommnemonic benchmarks the process of converting a cipher seed
+// (given the salt), to an enciphered mnemonic.
+func BenchmarkTomnemonic(b *testing.B) {
+	scryptN = 32768
+	scryptR = 8
+	scryptP = 1
+
+	pass := []byte("1234567890abcedfgh")
+	cipherSeed, err := New(0, nil, time.Now())
+	if err != nil {
+		b.Fatalf("unable to create seed: %v", err)
+	}
+
+	var r Mnemonic
+	for i := 0; i < b.N; i++ {
+		r, err = cipherSeed.ToMnemonic(pass)
+		if err != nil {
+			b.Fatalf("unable to encipher: %v", err)
+		}
+	}
+
+	b.ReportAllocs()
+
+	mnemonic = r
+}
+
+// BenchmarkToCipherSeed benchmarks the process of deciphering an existing
+// enciphered mnemonic.
+func BenchmarkToCipherSeed(b *testing.B) {
+	scryptN = 32768
+	scryptR = 8
+	scryptP = 1
+
+	pass := []byte("1234567890abcedfgh")
+	cipherSeed, err := New(0, nil, time.Now())
+	if err != nil {
+		b.Fatalf("unable to create seed: %v", err)
+	}
+
+	mnemonic, err := cipherSeed.ToMnemonic(pass)
+	if err != nil {
+		b.Fatalf("unable to create mnemonic: %v", err)
+	}
+
+	var s *CipherSeed
+	for i := 0; i < b.N; i++ {
+		s, err = mnemonic.ToCipherSeed(pass)
+		if err != nil {
+			b.Fatalf("unable to decipher: %v", err)
+		}
+	}
+
+	b.ReportAllocs()
+
+	seed = s
+}