Use directly FromStr types in StructOpt

[RCasatta]

With wrong values instead of getting:
[2021-09-21T13:25:13Z ERROR bdk_cli] Bip32(Base58(BadChecksum(1292914229, 1275556062)))

you get:
error: Invalid value for '--xprv ': base58 encoding error: base58ck checksum 0x4c0770de does not match expected 0x4d104e35

Description

Notes to the reviewers

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature
• I've updated CHANGELOG.md

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

LGTM, just need to fix test::test_key_derive, I think this is all it needs:

let key_generate_cmd = KeySubCommand::Derive {
    xprv: ExtendedPrivKey::from_str("tprv8ZgxMBicQKsPfQjJy8ge2cvBfDjLxJSkvNLVQiw7BQ5gTjKadG2rrcQB5zjcdaaUTz5EDNJaS77q4DzjqjogQBfMsaXFFNP3UqoBnwt2kyT").unwrap(),
    path: DerivationPath::from_str("m/84'/1'/0'/0").unwrap(),
};

[notmandatory]

I also noticed that I'm getting the same output on a checksum error, even so I think this change is an improvement code-wise.

cargo run -- key derive --xprv "tprv8ZgxMBicQKsPfQjJy8ge2cvBfDjLxJSkvNLVQiw7BQ5gTjKadG2rrcQB5zjcdaaUTz5EDNJaS77q4DzjqjogQBfMsaXFFNP3UqoBnwt2ky" --path "m/84'/1'/0'/0" 
    Finished dev [unoptimized + debuginfo] target(s) in 0.05s
     Running `target/debug/bdk-cli key derive --xprv tprv8ZgxMBicQKsPfQjJy8ge2cvBfDjLxJSkvNLVQiw7BQ5gTjKadG2rrcQB5zjcdaaUTz5EDNJaS77q4DzjqjogQBfMsaXFFNP3UqoBnwt2ky --path 'm/84'\''/1'\''/0'\''/0'`
error: Invalid value for '--xprv <XPRV>': base58 encoding error: base58ck checksum 0x125377bb does not match expected 0x5565d084

I think because StructOpt uses fmt::Display on any Error before displaying it, but that provides more human readable info so is a good thing.

[RCasatta]

I also noticed that I'm getting the same output on a checksum error

The error message is improved because it displays which CLI parameter is wrong, which is very useful I think

[notmandatory]

The error message is improved because it displays which CLI parameter is wrong, which is very useful I think

Ah yes you're right, I wasn't in an old branch when I tested. This PR does produce a much better error message.

[notmandatory]

ACK 134cc06