Copy go-bip39 into internal/bip39 (closes #42)

go-bip39's repository no longer exists, so keyfunc now carries the
part of v1.1.0 it uses, with the upstream LICENSE beside it, and
imports it from internal/bip39. Upstream's tests and test vectors for
the kept code come along unchanged; where they called a removed
function, crypto/rand stands in for NewEntropy and EntropyFromMnemonic
for MnemonicToByteArray.

Changes beyond the trimming are what the linter asked for: the
package-level variables moved into the functions that use them, three
error strings were lower-cased, and the unknown-word error now wraps
ErrInvalidMnemonic.

go.mod no longer requires go-bip39. go mod tidy keeps its two go.sum
lines, because a test in sneak/secret's bip85 package imports it, and
drops six lines that only go-bip39's own requirements needed.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

internal/bip39/english_internal_test.go

@@ -0,0 +1,19 @@
+package bip39
+
+import (
+	"hash/crc32"
+	"testing"
+)
+
+func TestEnglishChecksum(t *testing.T) {
+	t.Parallel()
+
+	// Ensure word list is correct
+	// $ wget https://raw.githubusercontent.com/bitcoin/bips/master/bip-0039/english.txt
+	// $ crc32 english.txt
+	// c1dbd296
+	checksum := crc32.ChecksumIEEE([]byte(english))
+	if checksum != 0xc1dbd296 {
+		t.Error("english checksum invalid")
+	}
+}