feat(ident): normalise names into C macro identifiers

Uppercases, replaces non-alphanumerics with underscores, and derives the
HAVE_X and SIZEOF_X forms the generated header uses.

lib/ident.ml

@@ -0,0 +1,18 @@
+let cident s =
+  let b = Buffer.create (String.length s + 4) in
+  String.iter
+    (fun c ->
+      match c with
+      | 'a' .. 'z' | 'A' .. 'Z' | '0' .. '9' -> Buffer.add_char b (Char.uppercase_ascii c)
+      | '*' -> Buffer.add_string b "_P"
+      | _ -> Buffer.add_char b '_')
+    s;
+  let raw = Buffer.contents b in
+  let parts = String.split_on_char '_' raw |> List.filter (fun p -> p <> "") in
+  String.concat "_" parts
+
+let lower s = String.lowercase_ascii (cident s)
+let have s = "HAVE_" ^ cident s
+let have_var s = "have_" ^ lower s
+let sizeof_name s = "SIZEOF_" ^ cident s
+let sizeof_var s = "sizeof_" ^ lower s