build: rename project "ledger"

[adamdecaf]

Accounts (as a service) is confusing as this project is more so designed to be a ledger.

Account (as a model) is confusing as Customers (service) has an Account type as well.

Let's rename Account to Instrument? As-in Financial Instrument.

cc @wadearnold @joshsadler

[adamdecaf]

Link dump

• https://developer.squareup.com/blog/books-an-immutable-double-entry-accounting-database-service/
• https://www.youtube.com/watch?v=iN6mhI5hFt4
• https://izakey.github.io/fineract-cn-api-docs-site/accounting/
• https://github.com/RealImage/QLedger