adding opt_map to simplify result function application over optionals

[ewenmaclean]

A function I use a lot when applying CCResult over optionals - e.g. in a translation of ASTs a fragment like:

| AST_from.Return expr -> let+ expr = opt_map trans_expr expr in AST_to.Return expr

where expr is an optional expression of type a option and trans_expr maps from a -> ('b,'err) result

[c-cube]

Thank you! 🕺