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/mnemonic/mnemonic.go

@@ -10,8 +10,8 @@ import (
 	"os/exec"
 	"strings"
 
-	bip39 "github.com/tyler-smith/go-bip39"
 	"golang.org/x/term"
+	"sneak.berlin/go/keyfunc/internal/bip39"
 )
 
 const (