test: migrate 700_e2e rows to corpus fixtures; retire the carrier

136 rows migrated, 0 already owned, 3 wwstage-failing rows held out.

test/wcc/data/r700_struct_i32_fields/case.ww

@@ -0,0 +1,9 @@
+//ww:run-exit 25
+// Migrated from 700_e2e row 26.
+package main;
+type point = struct { x: i32, y: i32 };
+fn distsq(p: point) i32 = { return p.x * p.x + p.y * p.y; };
+fn main() i32 = {
+  let p: point = point { x = 3, y = 4 };
+  return distsq(p);
+};