core: can't decode an encoded peer.ID, for certain peer ID values

Nobody has claimed this yet.

Minimal example:

```go
package main

import (
"log"

"github.com/libp2p/go-libp2p-core/peer"
)

func main() {
data := []byte{
0x00, 0x01, 0x72, 0x94, 0xe4, 0xc5, 0x30, 0x1b, 0x30, 0x30, 0x30, 0x0e, 0x30, 0xbd, 0xef,
0x30, 0x30, 0x30, 0x22, 0x77, 0xff, 0x22, 0x30, 0xef, 0x30, 0x30, 0x30, 0xbd, 0x30, 0x30, 0x30,
0x30, 0xbd, 0x30, 0x30,
}
var id peer.ID
if err := id.UnmarshalText(data); err != nil {
log.Fatal("unmarshal failed")
}
encoded := peer.Encode(id)
id2, err := peer.Decode(encoded)
if err != nil {
log.Fatal(err)
}
if id != id2 {
log.Fatal("expected ids to match")
}
}
```

The `peer.Decode(encoded)` step here fails: `failed to parse peer ID: expected 1 as the cid version number, got: 3261359138256`.

As far as I can tell, the reason for the failure is that `Encode` always uses base58 encoding: https://github.com/libp2p/go-libp2p-core/blob/83ac1d370dfe3f04aee38a904f73e49070be4863/peer/peer.go#L144-L146
Encoding uses the CID code to decode, if the encoded value doesn't start with "1" or "Qm": https://github.com/libp2p/go-libp2p-core/blob/83ac1d370dfe3f04aee38a904f73e49070be4863/peer/peer.go#L172-L187

I'm not sure what to do here. Can someone with more IPLD experience help me out here? @rvagg, @warpfork?

Start with peer/peer.go at the Encode and Decode implementations referenced in the issue, then run the minimal Go example to reproduce the round-trip failure. Trace how the encoded peer ID is interpreted when it does not begin with "1" or "Qm". Done means the example decodes successfully and the original and decoded peer.ID values match.

Written by the indexing model from the issue text.