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_bang_variants_remap/case.ww

@@ -0,0 +1,23 @@
+//ww:run-exit 107
+// Migrated from 700_e2e row 68.
+package main;
+type invalid = !i32;
+type overflow = !void;
+fn inner(n: i32) (invalid | i64 | overflow) = {
+  if (n == 0) { return 7: invalid; };
+  if (n < 0)  { return void: overflow; };
+  return n: i64 + 1000;
+};
+fn outer(n: i32) (overflow | i64 | invalid) = {
+  let v: i64 = inner(n)?;
+  return v + 1;
+};
+fn main() i32 = {
+  let r: (overflow | i64 | invalid) = outer(0);
+  match (r) {
+    case let v: i64 => return v: i32;
+    case let e: invalid => return e + 100;
+    case let e: overflow => return 999;
+  };
+  return 0;
+};