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_arr_u8_rw/case.ww

@@ -0,0 +1,14 @@
+//ww:run-exit 10
+// Migrated from 700_e2e row 6.
+package main;
+fn main() i32 = {
+  let buf: [4]u8;
+  buf[0] = 1: u8;
+  buf[1] = 2: u8;
+  buf[2] = 3: u8;
+  buf[3] = 4: u8;
+  let sum: i32 = 0;
+  let i: i32 = 0;
+  for (i < 4) { sum += buf[i]: i32; i += 1; };
+  return sum;
+};