While #4373 (closed) permits

Prelude> let v₁ = 1

the following is rejected

Prelude> let m >>=₁ f = undefined

<interactive>:0:10: lexical error at character '\8321'

Identifiers with non-numeric subscripts are not accepted either:

Prelude> let vₐ = 1

<interactive>:0:6: lexical error at character '\8336'

I wrote a small patch that makes such definitions possible.

1. A new unicode Alex macro, $subsup, is introduced and added to $idchar, $symchar, and $graphic
2. A unicode code point is classified as $subsup by alexGetChar iff either of the following holds:

a. The code point is annotated with <sub> or <super> in http://www.unicode.org/Public/UNIDATA/UnicodeData.txt

b. It is the [DOUBLE/TRIPLE/QUADRUPLE] PRIME (U+2032, U+2033, U+2034, U+2057)