Switch from BatEnum to Seq

[karoliineh]

A try on switching to the more standard Seq.t in Enum instances where possible as proposed in #1437

Some things that I noticed:

• Enum.cartesian_product is called product in Seq
• Enum.uncombine is called unzip (or split) in Seq
• Enum.fold is called fold_left in Seq
• There is no corresponding function to Enum.uniq in Seq
• There is no to_seq in BatHashtbl
• There is no to_seq_keys in BatHashtbl as in the Ocaml standard library Hashtbl

TODO

• Fix Gobview.
• Wait for Add reset, find_opt and Seq functions to BatHashtbl ocaml-batteries-team/batteries-included#1133 to release?
• Bump OCaml lower bound to 4.14? Many Seq functions are that new. (Bump OCaml lower bound to 4.14 #1448)