Add solana-keygen

[garious]

The problem:

solana-mint > id.json
solana-wallet airdrop --tokens=123 --mint=id.json
solana-fullnode-config > validator.json     <---- doh! not the keypair from id.json

Related mess: We're using solana-mint as identity files because it happens to generate new keypairs. Then in solana-wallet, we call the parameter --mint because it's in the Mint JSON format and otherwise has no relationship to the Mint's file format.

There should be no solana-mint and instead just a solana-keygen. To get a token count to solana-genesis, we'd pass that in as a CLI option instead of through mint.json.

By default, solana-keygen should create ~/.config/solana/id.json and include a CLI option to change that path. The format of the file could either be Vec[u8] or a base58-encoded string.

Likewise, by default, solana-wallet and solana-fullnode should look for id.json in the home directories. solana-genesis, on the other hand, should point to a different location explicitly for the Mint's identity file.

Here's what the new workflow would look like:

solana-keygen
solana-wallet airdrop --tokens=123
solana-fullnode-config > validator.json     <---- w00t! the keypair from ~/.config/solana/id.json

And for genesis:

solana-keygen -o - | solana-genesis -o genesis.log