JRFC 17 - <base>b<digits> -- ascii string encoding base prefixes

When ascii encoding numbers of different bases, we either (a) signal elsewhere what the base is (non self describing and to be avoided), or (b) prefix the characters with a base identifier:

```
0b010101 binary
0xabcdef hex
```

But what about other bases? base32, base64, base58 are all popular. What about this encoding:

```
b
2b010101 binary
16babcdef hex
32b123wer base32
58b01AaBb base58
64bAaBb-_ base64
```
- flexible to any base
- completely self-describing
- no need to maintain a well-known table of identifiers
- (still need to maintain a table of alphabets)