Require OCaml >= 4.10

[tahina-pro]

Johannes Blaser of @vusec reports F* not building on OCaml 4.08:

File "fstar-lib/generated/FStar_Monotonic_Seq.ml", lines 124-132, characters 2-46:
124 | ..fun l ->
125 |     fun max ->
126 |       fun i ->
127 |         fun init ->
128 |           fun log ->
129 |             FStar_HyperStack_ST.recall log;
130 |             FStar_HyperStack_ST.recall_region ();
131 |             FStar_HyperStack_ST.mr_witness () () () (Obj.magic log) ();
132 |             FStar_HyperStack_ST.ralloc () init
Error: This definition has type
         Prims.unit ->
         Prims.nat ->
         Prims.unit ->
         Prims.nat ->
         (Prims.unit, 'a) log_t ->
         (Prims.unit, 'a, Prims.unit, Prims.unit, Prims.unit) seqn
       which is less general than
         'a0.
           Prims.unit ->
           Prims.nat ->
           Prims.unit ->
           Prims.nat ->
           (Prims.unit, 'a0) log_t ->
           (Prims.unit, 'a0, Prims.unit, Prims.unit, Prims.unit) seqn
make: *** [Makefile:23: dune-fstar] Error 1

I can reproduce the same failure with OCaml 4.09. By contrast, OCaml 4.10 succeeds.

So this PR updates the opam package definition and the documentation, and removes some file that was necessary for earlier versions of OCaml.