Loop unrolling during optimization

## Abstract

I propose adding support for loop unrolling as a compiler optimization step.

## Motivation

In the protocol I am working on, many operations are repeated a fixed number of times. For example:
```
uint256 constant _TOKEN_CNT = 6;

uint256[_TOKEN_CNT] memory calculatedData;

for (uint i = 0; i < _TOKEN_CNT; i++) {
calculatedData[i] = ...
}
```
All such loops in my code have a small bound, known at compile-time. Manually unrolling them results in gas savings, but it makes the code less maintainable, and more error-prone to silly mistakes if I want to deploy with different values of `_TOKEN_CNT`.

## Specification

The compiler could decide which constant-bound loops to unroll based on the body of the loop and the `runs` parameter of the optimization settings. Even better, it would be amazing if I could hint to the compiler that I want specific loop(s) to be unrolled no matter what.

## Backwards Compatibility

I don't think this would bring any breaking changes, but a simple toggle to disable loop unrolling altogether would make this fully backwards compatible.

No files, tests, or entry points are identified. Start by examining the compiler optimization pipeline and how the runs parameter is handled; done would mean defining and validating a supported approach for unrolling constant-bound loops, including the requested control over unrolling.

Written by the indexing model from the issue text.