feat: allow doSeq in let x <- e | seq

[digama0]

By changing let x <- e | seq to allow a sequence of doElems at seq instead of just one, it fixes #1804. The syntax extension is also convenient in some cases.

[Kha]

Nice generalization. I assume it's not trivial to trigger a sequence unintentionally unless you do something like

let x <- a | b
             c

? Seem fine to me then.

[digama0]

I would use the formatting

let x <- a
  | b
    c

if I actually wanted a sequence, which I think looks clear enough. (You can also hang the | on the previous line if you want to use only one indent level - this is the style formatTerm uses. It is not allowed to use no indent for the | though.) We should probably be using colEq for the do parser though, since currently the above parses as if it was

let x <- a
  | b
c

which seems very misleading.

[Kha]

Yes, colEq would be appropriate

[gebner]

Ideally, do would use sepByIndent like by etc. (which uses colEq internally). The reason I didn't do that myself is because I didn't want to touch the do notation expander, but you could give it a try.