Fix Invalid Private Key Generation

[witoff]

Fixes #10 by upgrading btcutil dependencies.

This issue was tracked in the upstream btcsuite/btcutil dependency at btcsuite/btcutil#172

When a child key has less than 32 bytes, it is incorrectly padded leading to incorrect key derivation. This effects 1/256 hardened key derivations and twice is many (1/128) accounts with two hardened component. Upgrading dependencies replaces key.Child with key.Derive which correctly pads addresses.

Note that merging this will fix derivation, but break existing implementations for ~1/128 keys. btcsuite addressed this by adding a IsAffectedByIssue172() bool function and changing Child() to Derive() to break the default interface and force implementers to address as needed and I suggest taking a similar approach.

[miguelmota]

Hi @witoff! Thank you for the detailed explanation and PR. Looks good and happy to merge it. I like your suggestion of adding a DeriveNonStandard method to keep backward compatibility.

[witoff]

@miguelmota updated so that current functionality doesn't break. Since this is used by many upstream libraries, users will want a way to enable correct behavior without requiring everyone else to update so i also added envar support w/ tests.

// This works as it did before.
account, err := wallet.Derive(path, false)

// This derives correctly
os.Setenv(issue179FixEnvar, "1")
account, err = wallet.Derive(path, false)

// This also derives correctly
wallet.SetFixIssue172(true)
account, err = wallet.Derive(path, false)

Can you merge or advise on any other changes you think are needed?

[miguelmota]

@witoff This is perfect! Awesome work. Merging.