Add user option to preserve the order of imenu candidates

[pacastega]

This commit adds a new user option haskell-decl-scan-sort-imenu to decide whether to sort the candidates in imenu (done in function haskell-ds-create-imenu-index).

When t (the default), sort the candidates alphabetically as before. When nil, present them in the same order as they appear in the file (note that this requires reversing the list of candidates, since it is built in such a way that the canditates appearing later in the file are appended to the front of the list).

[purcell]

Makes sense, merging, thanks.