Convert span's start to line number + col

This requires to make line numbers a first-class citizen in the module
  hierarchy but it is fortunately not _too involved_.

crates/aiken-lang/src/snapshots/can_handle_comments_at_end_of_file.snap

@@ -19,5 +19,17 @@ Module {
             },
         ),
     ],
+    lines: LineNumbers {
+        line_starts: [
+            0,
+            10,
+            11,
+            27,
+        ],
+        length: 43,
+        last: Some(
+            27,
+        ),
+    },
     kind: Validator,
 }

crates/aiken-lang/src/snapshots/function_ambiguous_sequence.snap

@@ -197,5 +197,32 @@ Module {
             },
         ),
     ],
+    lines: LineNumbers {
+        line_starts: [
+            0,
+            13,
+            27,
+            34,
+            36,
+            37,
+            50,
+            64,
+            71,
+            73,
+            74,
+            87,
+            104,
+            106,
+            107,
+            120,
+            138,
+            154,
+            156,
+        ],
+        length: 156,
+        last: Some(
+            156,
+        ),
+    },
     kind: Validator,
 }

crates/aiken-lang/src/snapshots/parse_unicode_offset_1.snap

@@ -48,5 +48,18 @@ Module {
             },
         ),
     ],
+    lines: LineNumbers {
+        line_starts: [
+            0,
+            11,
+            27,
+            31,
+            33,
+        ],
+        length: 33,
+        last: Some(
+            33,
+        ),
+    },
     kind: Validator,
 }

crates/aiken-lang/src/snapshots/parse_unicode_offset_2.snap

@@ -46,5 +46,18 @@ Module {
             },
         ),
     ],
+    lines: LineNumbers {
+        line_starts: [
+            0,
+            11,
+            25,
+            29,
+            31,
+        ],
+        length: 31,
+        last: Some(
+            31,
+        ),
+    },
     kind: Validator,
 }

crates/aiken-lang/src/snapshots/windows_newline.snap

@@ -20,5 +20,15 @@ Module {
             },
         ),
     ],
+    lines: LineNumbers {
+        line_starts: [
+            0,
+            16,
+        ],
+        length: 16,
+        last: Some(
+            16,
+        ),
+    },
     kind: Validator,
 }