[fidl][cpp] Use std:: types for non-nullable strings and vectors.

See: https://fuchsia-review.googlesource.com/c/garnet/+/236996

Test: built everything, CQ, booted and poked around

Change-Id: Ie2a50b80aefd5ab6ec4c62d14653badb90e0bb4f
diff --git a/util/env_fuchsia.cc b/util/env_fuchsia.cc
index 7f6689b..bcaedf1 100644
--- a/util/env_fuchsia.cc
+++ b/util/env_fuchsia.cc
@@ -461,7 +461,7 @@
     while (bytes_read < n) {
       size_t step = std::min(n - bytes_read, fuchsia::io::MAX_BUF);
       zx_status_t read_status;
-      fidl::VectorPtr<uint8_t> read_data;
+      std::vector<uint8_t> read_data;
       zx_status_t fidl_status =
           file_->ReadAt(step, offset + bytes_read, &read_status, &read_data);
       if (fidl_status != ZX_OK) {
@@ -470,15 +470,15 @@
       if (read_status != ZX_OK) {
         return FuchsiaZxError(filename_, read_status);
       }
-      if (read_data->size() > step) {
+      if (read_data.size() > step) {
         return Status::IOError(filename_, "Received more bytes than read");
       }
-      if (read_data->size() == 0) {
+      if (read_data.size() == 0) {
         break;
       }
-      memcpy(scratch + bytes_read, reinterpret_cast<char*>(read_data->data()),
-             read_data->size());
-      bytes_read += read_data->size();
+      memcpy(scratch + bytes_read, reinterpret_cast<char*>(read_data.data()),
+             read_data.size());
+      bytes_read += read_data.size();
     }
     *result = Slice(scratch, bytes_read);
     return Status::OK();
@@ -831,25 +831,25 @@
     if (!s.ok()) {
       return s;
     }
-    fidl::VectorPtr<uint8_t> buf;
+    std::vector<uint8_t> buf;
     zx_status_t fidl_status;
     zx_status_t read_status;
     vdirent_t entry;
     while ((fidl_status = d->ReadDirents(fuchsia::io::MAX_BUF, &read_status,
                                          &buf)) == ZX_OK &&
-           read_status == ZX_OK && buf->size() > 0) {
+           read_status == ZX_OK && buf.size() > 0) {
       size_t offset = 0;
-      while (offset + sizeof(vdirent_t) <= buf->size()) {
-        memcpy(&entry, buf->data() + offset, sizeof(vdirent_t));
-        if (sizeof(vdirent_t) + entry.size > buf->size() - offset) {
+      while (offset + sizeof(vdirent_t) <= buf.size()) {
+        memcpy(&entry, buf.data() + offset, sizeof(vdirent_t));
+        if (sizeof(vdirent_t) + entry.size > buf.size() - offset) {
           break;
         }
         result->push_back(
-            std::string((char*)buf->data() + offset + sizeof(vdirent_t),
+            std::string((char*)buf.data() + offset + sizeof(vdirent_t),
                         (size_t)entry.size));
         offset += sizeof(vdirent_t) + entry.size;
       }
-      if (offset != buf->size()) {
+      if (offset != buf.size()) {
         return Status::IOError(dir, "Malformed dirent");
       }
     }