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_sum_param_u8_slice/case.ww

@@ -0,0 +1,23 @@
+//ww:run-exit 40
+// Migrated from 700_e2e row 125.
+package main;
+import bytes;
+fn main() i32 = {
+  let buf: [4]u8;
+  buf[0] = 1u8; buf[1] = 2u8; buf[2] = 3u8; buf[3] = 4u8;
+  let needle: [2]u8;
+  needle[0] = 3u8; needle[1] = 4u8;
+  let r1: (i32 | void) = bytes.index(buf[0:4], 3u8);
+  let r2: (i32 | void) = bytes.index(buf[0:4], needle[0:2]);
+  let a: i32 = 99;
+  let b: i32 = 99;
+  match (r1) {
+    case let i: i32 => a = i;
+    case void => a = -1;
+  };
+  match (r2) {
+    case let i: i32 => b = i;
+    case void => b = -1;
+  };
+  return a * 10 + b + 18;
+};