Add a Span constructor to SizePrefixedFlatbufferVector

This lets us copy without duplicating the copy code all over the place.

Change-Id: I17e8746ad1186c833a82167fee06efa4c0ffad42
Signed-off-by: Austin Schuh <austin.schuh@bluerivertech.com>
diff --git a/aos/events/logging/logfile_utils.cc b/aos/events/logging/logfile_utils.cc
index f32b337..5f0e372 100644
--- a/aos/events/logging/logfile_utils.cc
+++ b/aos/events/logging/logfile_utils.cc
@@ -411,10 +411,7 @@
   }
 
   // And copy the config so we have it forever, removing the size prefix.
-  ResizeableBuffer data;
-  data.resize(config_data.size());
-  memcpy(data.data(), config_data.begin(), data.size());
-  SizePrefixedFlatbufferVector<LogFileHeader> result(std::move(data));
+  SizePrefixedFlatbufferVector<LogFileHeader> result(config_data);
   if (!result.Verify()) {
     return std::nullopt;
   }
@@ -435,10 +432,7 @@
   }
 
   // And copy the config so we have it forever, removing the size prefix.
-  ResizeableBuffer data;
-  data.resize(data_span.size());
-  memcpy(data.data(), data_span.begin(), data.size());
-  SizePrefixedFlatbufferVector<MessageHeader> result(std::move(data));
+  SizePrefixedFlatbufferVector<MessageHeader> result(data_span);
   if (!result.Verify()) {
     return std::nullopt;
   }
@@ -480,10 +474,7 @@
     return std::nullopt;
   }
 
-  ResizeableBuffer result_buffer;
-  result_buffer.resize(msg_data.size());
-  memcpy(result_buffer.data(), msg_data.begin(), result_buffer.size());
-  SizePrefixedFlatbufferVector<MessageHeader> result(std::move(result_buffer));
+  SizePrefixedFlatbufferVector<MessageHeader> result(msg_data);
 
   const monotonic_clock::time_point timestamp = monotonic_clock::time_point(
       chrono::nanoseconds(result.message().monotonic_sent_time()));