Billing: Order, Delivery, and Payment

[samlown]

• Renamed old bill.Payment and bill.Delivery types to bill.PaymentDetails and bill.DeliveryDetails, so that the names are available for the new document types.
• Renamed bill.Receipt to bill.Payment as the new base type for communicating payments.

Pre-Review Checklist

• I have performed a self-review of my code.
• I have added thorough tests with at least 90% code coverage.
• I've modified or created example GOBL documents to show my changes in use, if appropriate.
• When adding or modifying a tax regime or addon, I've added links to the source of the changes, either structured or in the comments.
• I've run go generate . to ensure that the Schemas and Regime data are up to date.
• All linter warnings have been reviewed and fixed.
• I've been obsessive with pointer nil checks to avoid panics.
• The CHANGELOG.md has been updated with an overview of my changes.
• Requested a review from @samlown.

[codecov]

Codecov Report

Attention: Patch coverage is 95.63786% with 53 lines in your changes missing coverage. Please review.

Project coverage is 86.87%. Comparing base (a7b2e43) to head (776e78a).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
bill/calculator.go	89.14%	13 Missing and 6 partials ⚠️
bill/order.go	90.36%	12 Missing and 4 partials ⚠️
bill/delivery.go	94.18%	8 Missing and 2 partials ⚠️
bill/exchange.go	94.17%	4 Missing and 2 partials ⚠️
bill/line.go	98.21%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #462      +/-   ##
==========================================
+ Coverage   85.97%   86.87%   +0.89%     
==========================================
  Files         258      263       +5     
  Lines       12847    13430     +583     
==========================================
+ Hits        11045    11667     +622     
+ Misses       1454     1415      -39     
  Partials      348      348              

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[samlown]

Using pointers will require changes to current implementations of convertors, but is backwards compatible.