Size or length operation on Foldable

[benhutchison]

Please provide a size method, with the ordinary meaning eg:

@ Foldable[List].foldLeft(List(1, 2, 3), 0){case (count, elem) => count + 1}
res6: Int = 3

Both (a) to provide a convenient default impl for a common operation, and (b) to allow instances to define their own faster size computation.

[benhutchison]

background context from haskell: https://www.reddit.com/r/haskell/comments/xo05l/why_isnt_length_part_of_foldable/

[ceedubs]

👍. I've also wondered if we should have a similar method that would be safe on infinite streams - something like "size, but stop calculating if it's over N".

[ceedubs]

This was introduced in #1114.