[Feature] Limit Indexes and Loops to u32

[gluax]

🚀 Feature

Loops and Indexes should be limited to u32 types once type casting is in.

Motivation

As seen in #905, our type inference system is not symmetrical in all cases.
The reasoning to wait for typecasting is to cast the iterator part of a loop to your desired type.
If we want more types allowed in the future, we can redo the type inference algorithm, but that's more work and would be another feature.

Implementation

Just simply restrict allowed types.

[acoglio]

This was discussed at today's compiler sync, and there was consensus on this. The restriction can be relaxed in the future.

@Protryon @collinc97 @howardwu Do you have objections to this idea?

[gluax]

Blocked by #960.

[collinc97]

As of Leo v.1.62, we have implemented magnitude for loop bounds requiring a u8, u16, u32 type.