refactor: normalize for loop declarations

[0xJabberwock]

Description

Previously, I raised a concern about the possible gas savings that could be achieved by prefix incrementing or decrementing the iterator variable within declarations of for loops (see #468 (comment); #475). Anyways, after having implemented the suggestion, the gas snapshots didn't change.

Consequently, I researched and found out that compiling via_ir = true already applies this optimization and so equals the gas expenditures, but the same doesn't necessarily happen when using the legacy pipeline (see ethereum/solidity#14595). Another reason for the unaltered gas snapshots may be that the savings are negligible (less than 1 % change).

Moreover, explicitly setting the iterator variable to its default value while declaring it may also be a cause of increased gas consumption. Again, I haven't seen modifications in the gas snapshots after having applied this locally, so I opted for leaving such explicit initialization momentarily as it appeared to be enforced for every loop, despite it could be not. Has this been done on purpose?

Ultimately, in relation with the for comparisons, I tried cacheing the length of the arrays being iterated over and, similarly, the gas savings were either negligible or none.

Apart from that, this PR aims to standardize every Solidity for loop declaration across the codebase. For so, the iterator variable has been named i rather than the less frequent and longer index, and prefix increments and decrements have been enforced where applicable. Normalizing the comparisons by cacheing didn't seem feasible due to the ad hoc nature of each logic and stack too deep concerns.

Type of change

• Bug fix
• New feature
• Breaking change
• Dependency changes
• Code refactor / cleanup
• Documentation or wording changes
• Other

Checklist:

• The diff is legible and has no extraneous changes
• Complex code has been commented, including external interfaces
• Tests have 100% code coverage
• The base branch is either main, or there's a description of how to merge

Issue Resolution

Closes #475.

[wei3erHase]

looks good, i don't have strong opinions on ++i vs i++, but i do prefer standarizing i over index.

[EndymionJkb]

Not sure how the "index" ones snuck in there; we did have a policy to always use "i", "j", ..., unless the name is semantic (e.g., pathIndex, stepIndex or something), and would be confusing to be a single letter. A straight-up iterator should always be i,j,k,...