Cleaning, documentation, uniformisation of the Coq extension of List.

Still some discrepancies though. E.g.:
- some functions taking an equality as arguments have suffix `_f` but
  not all;
- the functions possibly raising an error have still different kinds
  of failure (Failure, Invalid_argument, Not_found or IndexOutOfRange,
  and when in the first two cases, with no unique rules in the style
  of the associated string - we thus avoid to document the exact
  string used).

There are a few semantics changes:
- skipn_at_least now raises a `Failure` if its argument is negative;
- map3 raises an Invalid_argument "List.map3" rather than
  Invalid_argument "map3" and similarly for map4
- internally, map3 and map4 are now tail-recursive (by uniformity);
- internally, split3 and combine3 are now tail-recursive (by uniformity);
- filter is now "smart" by default and smartfilter is deprecated;
- smartmap is now tail-recursive by default.

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