Obtain struct layout details from C to help avoid runtime mismatches.

```ocaml
   # module V = Verify_struct_layout
   [...]
   # V.struct_info ~headers:["sys/types.h"] ~tag:"timeval"
       ~fields:["tv_sec"; "tv_usec"];;
   - : V.struct_info =
   {V.tag = "timeval";
    fields =
     [{V.label = "tv_sec"; offset = 0; size = 8};
      {V.label = "tv_usec"; offset = 8; size = 8}];
    size = 16}
   # V.struct_info ~headers:["time.h"] ~tag:"tm"
       ~fields:["tm_isdst"; "tm_mday"; "tm_mon"; "tm_year";
                "tm_wday"; "tm_yday"; "tm_sec"; "tm_min"; "tm_hour"];;
   - : V.struct_info =
   {V.tag = "tm";
    fields =
     [{V.label = "tm_sec"; offset = 0; size = 4};
      {V.label = "tm_min"; offset = 4; size = 4};
      {V.label = "tm_hour"; offset = 8; size = 4};
      {V.label = "tm_mday"; offset = 12; size = 4};
      {V.label = "tm_mon"; offset = 16; size = 4};
      {V.label = "tm_year"; offset = 20; size = 4};
      {V.label = "tm_wday"; offset = 24; size = 4};
      {V.label = "tm_yday"; offset = 28; size = 4};
      {V.label = "tm_isdst"; offset = 32; size = 4}];
    size = 56}
```
Currently just a comically inefficient proof-of-concept implementation.