UvisorBoxIndex should wrap all uVisor internal structures

We should wrap the RPC structure inside `UvisorBoxIndex` to improve the `UvisorBssSection` abstraction.

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`UvisorBssSections` contains structures that can be accessed by user-space code. Space for these structs is allocated at runtime in each box private BSS section. The user can then access the sections via dedicated pointers.

At the moment the structure exposes the `UvisorBoxIndex` struct, the RPC struct, the private context and the private (static) heap. While for the private context and heap it makes sense to expose a separate pointer (we expect the user to access them individually), the RPC struct is for internal use only and should not be exposed.

**Note**: This is just a UX/abstraction issue. The user-space code can still access the RPC structure.

No source file or test is named. Start by locating UvisorBssSections and UvisorBoxIndex, then trace how the RPC structure, private context, and private heap are exposed to user-space code. Done means the RPC structure is internal to the abstraction while the intended context and heap pointers remain accessible, without changing RPC behavior.

Written by the indexing model from the issue text.