Change slice::to_vec to not use extend_from_slice

This also required adding a loop guard in case clone panics

Add specialization for copy

There is a better version for copy, so I've added specialization for that function
and hopefully that should speed it up even more.

Switch FromIter<slice::Iter> to use `to_vec`

Test different unrolling version for to_vec

Revert to impl

From benchmarking, it appears this version is faster

src/test/codegen/to_vec.rs

@@ -0,0 +1,10 @@
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @copy_to_vec
+#[no_mangle]
+fn copy_to_vec(s: &[u64]) -> Vec<u64> {
+  s.to_vec()
+  // CHECK: call void @llvm.memcpy
+}