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_strconv_stou64/case.ww

@@ -0,0 +1,21 @@
+//ww:run-exit 130
+// Migrated from 700_e2e row 99.
+package main;
+import strconv;
+type r_t = (u64 | strconv.invalid | strconv.overflow);
+fn main() i32 = {
+  let r1: r_t = strconv.stou64("123", strconv.base.DEC);
+  let r2: r_t = strconv.stou64("-1", strconv.base.DEC);
+  let acc: i32 = 0;
+  match (r1) {
+    case let v: u64 => acc += v: i32;
+    case let e: strconv.invalid => acc += -100;
+    case let e: strconv.overflow => acc += -200;
+  };
+  match (r2) {
+    case let v: u64 => acc += 100;
+    case let e: strconv.invalid => acc += 200;
+    case let e: strconv.overflow => acc += 7;
+  };
+  return acc;
+};