style(Core) : a₁ and a₂ instead of a1 and a2

[madvorak]

style: Core/Init/Subtype.eq
Name arguments idiomatically a₁ and a₂ instead of a1 and a2

[nomeata]

Thanks for trying to make lean better. However, I don't think a change like this has enough impact to justify the work that goes into it (reading, reviewing, merging, wondering about whether this is a breaking change for users who explicitly specify arguments), so I'll take the liberty to reject this one.

Maybe in the future we'll have and enforce a style guide, and fix issues like these at efficiently scale.