Copy go-bip39 into internal/bip39 (closes #122)

go-bip39's repository no longer exists. The part of v1.1.0 that secret
uses (NewEntropy, NewMnemonic, NewSeed, IsMnemonicValid and what they
call; English word list only) now lives in internal/bip39 with
upstream's LICENSE beside it, along with upstream's tests of that code
and their vectors unchanged. Every import moves there, and the module
leaves go.mod and go.sum. Beyond the trimming, only what the linter
asked for changed. No derived key or mnemonic changes.

Model: opus-5-5

internal/bip39/example_test.go

@@ -0,0 +1,30 @@
+package bip39_test
+
+import (
+	"encoding/hex"
+	"fmt"
+
+	"sneak.berlin/go/secret/internal/bip39"
+)
+
+//nolint:lll // the test vector and its output, kept as upstream wrote them
+func ExampleNewMnemonic() {
+	// the entropy can be any byte slice, generated how pleased,
+	// as long its bit size is a multiple of 32 and is within
+	// the inclusive range of {128,256}
+	entropy, _ := hex.DecodeString("066dca1a2bb7e8a1db2832148ce9933eea0f3ac9548d793112d9a95c9407efad")
+
+	// generate a mnemomic
+	mnemomic, _ := bip39.NewMnemonic(entropy)
+	fmt.Println(mnemomic)
+	// output:
+	// all hour make first leader extend hole alien behind guard gospel lava path output census museum junior mass reopen famous sing advance salt reform
+}
+
+//nolint:lll // the test vector and its output, kept as upstream wrote them
+func ExampleNewSeed() {
+	seed := bip39.NewSeed("all hour make first leader extend hole alien behind guard gospel lava path output census museum junior mass reopen famous sing advance salt reform", "TREZOR")
+	fmt.Println(hex.EncodeToString(seed))
+	// output:
+	// 26e975ec644423f4a4c4f4215ef09b4bd7ef924e85d1d17c4cf3f136c2863cf6df0a475045652c57eb5fb41513ca2a2d67722b77e954b4b3fc11f7590449191d
+}