feat: internal manifest package

[letFunny]

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[letFunny]

Discussed offline with @niemeyer some parts of the PR. We agreed to:

• Keep duplicated lines in manifest.wall as "undefined behavior" because it means the file is malformed. We should not support it but for now we won't report an error.
• Change manifest.go to use pointers if it makes sense.

EDIT: I have done the tests for the second point and using pointers and a closure means that Go is allocating memory because of the escape analysis, i.e. it cannot figure out if the pointer will escape from the closure. To be discussed further, but keeping the structs for now.