Add Ordered file

[ihtefyw]

This PR adds the OrderedFile, an array-like KVFile.
Values have a fixed size, and their position in the file is determined by their key (uint64) times the size of the stored value.

This will be used to implement caching for the MPT root list

[codecov]

Codecov Report

❌ Patch coverage is 91.57895% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
go/backend/kv_file/ordered_file.go	91.57%	4 Missing and 4 partials ⚠️

Files with missing lines	Coverage Δ
go/backend/kv_file/ordered_file.go	91.57% <91.57%> (ø)

... and 2 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[HerbertJordan]

I think the implementation fits the intended use case. However, I remarked a few design issues that you might want to address. Please have a look.