owner and executable checks (#6526)

* owner_checks

* only system program may assign owner, and only if pre.owner is system

* moar coverage!

* moar coverage, allow re-assignment IFF data is zeroed

runtime/benches/is_zeroed.rs

@@ -0,0 +1,34 @@
+#![feature(test)]
+
+extern crate test;
+
+use solana_runtime::message_processor::is_zeroed;
+use test::Bencher;
+
+const BUFSIZE: usize = 1024 * 1024 + 127;
+static BUF0: [u8; BUFSIZE] = [0; BUFSIZE];
+static BUF1: [u8; BUFSIZE] = [1; BUFSIZE];
+
+#[bench]
+fn bench_is_zeroed(bencher: &mut Bencher) {
+    bencher.iter(|| {
+        is_zeroed(&BUF0);
+    });
+}
+
+#[bench]
+fn bench_is_zeroed_not(bencher: &mut Bencher) {
+    bencher.iter(|| {
+        is_zeroed(&BUF1);
+    });
+}
+
+#[bench]
+fn bench_is_zeroed_by_iter(bencher: &mut Bencher) {
+    bencher.iter(|| BUF0.iter().all(|item| *item == 0));
+}
+
+#[bench]
+fn bench_is_zeroed_not_by_iter(bencher: &mut Bencher) {
+    bencher.iter(|| BUF1.iter().all(|item| *item == 0));
+}