[Feature] Array variable indexing.

[d0cd]

🚀 Feature

Array indexing is restricted to constant u32s.
However given that arrays are fixed in size, it would be straightforward to implement array indexing by generating a sufficiently constrained circuit via an appropriate sequence of instructions.
The instructions should:

• check that the index is in bounds.
• mux the correct element of the array.

If this were to be implemented in snarkVM via an ARC, the Leo compiler could easily swap to the native version.