Implement a minimal "completion" service.
The use of Base48 encoding allows to efficiently implement a "completion" mechanism for blocks, operations, public key and contract identifiers. For instance:
```
> ./tezos-client complete eeHfgnr9QeDN
eeHfgnr9QeDNvcMgSfATNeDeec4KG4CkHHkNNJt5B9xdVmsxhsHNR
```
This command returns all the identifiers matching the given prefix. Adding the option `-unique` let the command fails when there more than one possible completion.
This completion mechanism should probably never be used implicitly, but it might still be useful to display a small completion popup in a GUI, or during manual testing on the CLI.
See merge request !101
