Enable 'confidential_assets' for Elements (following #66 and #317) to allow
deriving confidential addresses and signing confidential transactions.
The following Python helper script was used to update the JSON files:
```
import json
import sys
for f in sys.argv[1:]:
d = json.load(open(f))
d["confidential_assets"] = None
with open(f, "w") as o:
json.dump(d, o, indent=2)
o.write("\n")
```
Set it to `{'address_prefix': 4, 'blech32_prefix': 'el'}` for Elements.
`coins.json` and `coininfo.py` were re-generated using:
```
$ pipenv run make gen gen_check
```
