Add zeroed function to create a zeroed value of any type

[jfecher]

Problem

As explained in Issue #988, adding a std::zeroed function in Noir to return a zeroed value of any type would be useful to create "uninitialized" values for constructs like enums which guarantee these values will note be used.

Another possible usecase for such a function is adding a BoundedVec type to Noir which is backed by an array internally and has the array's size as its capacity. The array would initially be filled with N instances of zeroed::<T>() for some T, and these would be gradually replaced by the actual elements of the vector each time it is pushed to.

Proposed solution

We should add std::zeroed to our stdlib. Perhaps with a Note explaining why users should be cautious using it (using it on an arbitrary struct could create a struct with invalid invariants if it is used).

Alternatives considered

No response

Additional context

No response