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_embed/case.ww

@@ -0,0 +1,13 @@
+//ww:run-exit 42
+// Migrated from 700_e2e row 46.
+package main;
+type point = struct { x: i32, y: i32 };
+type vec = struct {
+  point,
+  struct { z: i32 },
+  w: i32,
+};
+fn main() i32 = {
+  let v: vec = vec { x = 1, y = 2, z = 3, w = 36 };
+  return v.x + v.y + v.z + v.w;
+};