Replace Ulex recommendation with Sedlex

[Leandros]

Sedlex is the successor to ulex and should be preferred.

Excerpt from the sedlex project page:

sedlex is the successor of the ulex project. Contrary to ulex which was implemented as a Camlp4 syntax extension, sedlex is based on the new "-ppx" technology of OCaml, which allow rewriting OCaml parse trees through external rewriters. (And what a better name than "sed" for a rewriter?)

(related #1286)

[XVilka]

Quite relevant, since there is a showstopper with camlp4 on the recent OCaml versions: https://discuss.ocaml.org/t/camlp5-and-ocaml-4-08/3985/12

See the list of the packages that still depend on ulex, thus cannot be built for OCaml 4.08 and higher versions:

• beluga - created an issue #108 for porting to sedlex
• cduce
• fstar>=0.9.5.0 - created an issue #1792 for porting to sedlex
• higlo - created an issue #3 for porting to sedlex
• kremlin - created an issue #138 for porting to sedlex
• lambdoc - created an issue #37 for porting to sedlex
• mezzo
• opa-base - created an issue #196 for porting to sedlex
• piqilib - there is an issue #59 for porting to sedlex
• pxp
• sociaml-vcard - created an issue #1 for porting to sedlex
• wdialog