Add assert in debug mode and tests

[elmattic]

Summary of changes
Changes introduced in this pull request:

• Add assert in debug mode to avoid misuses
• Add two tests

Reference issue to close (if applicable)

Closes #1361

Other information and links

Will notice PL so they can report PR on ref-fvm repo.

[LesnyRumcajs]

What do you think about using usize here? Unless we're supporting 32-bit systems it should be fine, plus we would get rid of some casts.

[elmattic]

Good question. We indeed want 32-bit systems support like wasm32. Protocol Labs had to port our code to explicitly use u64 (there are many commits, the one for Bitfield is here). We will probably use some of their crates instead of ours (I'm commiting here but this PR will have to be done on the ref-fvm repo too to keep things in sync in the meantime).

Even if we don't want to support say 32-bit systems I think it's a good practice to use properly sized types (makes the code more readable and correct).

[LesnyRumcajs]

Thanks for the explanation! In this case, should we rather do an opposite cast which wouldn't be lossy? Given that potentially (on a 32-bit machine) MAX_VARINT as usize would get truncated to 32 bits (u32::MAX). In the end the behaviour would still be the same - on a 32 bit machine the condition will always pass regardless.

[LesnyRumcajs]

Thanks for the explanation! In this case, should we rather do an opposite cast which wouldn't be lossy? Given that potentially (on a 32-bit machine) MAX_VARINT as usize would get truncated to 32 bits (u32::MAX). In the end the behaviour would still be the same - on a 32 bit machine the condition will always pass regardless.