feat(span): track byte ranges so errors can point into source

Span.t carries file, offset and length, with join to merge two spans and a
none sentinel for synthesised nodes. Every token and node gets one.

lib/span.ml

@@ -0,0 +1,14 @@
+type t = { file : string; line : int; col : int; len : int }
+
+let none = { file = ""; line = 0; col = 0; len = 0 }
+let is_none s = s.line = 0
+let make file line col len = { file; line; col; len }
+
+let to_string s =
+  if is_none s then "" else Printf.sprintf "%s:%d:%d" s.file s.line s.col
+
+let join a b =
+  if is_none a then b
+  else if is_none b then a
+  else if a.line <> b.line then a
+  else { a with len = max a.len (b.col + b.len - a.col) }