Also separated the roundtrip testing from the random string decoding for clarity
Note that while BIP 173 claims:
```
The human-readable part, which is intended to convey the type of data, or anything else that is relevant to the reader. This part MUST contain 1 to 83 US-ASCII characters, with each character having a value in the range [33-126]. HRP validity may be further restricted by specific applications.
```
bech32::Encode rejects uppercase letters.