Make all internal/private functions `static`

[iWas-Coder]

Describe the feature

In order to optimize the codebase as much as possible, we can define all internal/private funcions as static, so their visibility only spans inside its own translation unit.

Why should it be implemented? (optional)

In order to optimize the codebase as much as possible, also to make the code look as nice as it can get.

Do you think there is any alternative? (optional)

We'll also evaluate if they need to be inlinable or not, depending on its complexity.

Additional context (optional)

N/A

Extra questions (optional)

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• I'd like to work on this issue