Adds safe serial port write methods (#396)

The old method had a fairly large risk of undefined behavior, and the way the docs
were written could cause users to get confused.
Deprecate the old method and add StringRef method as preferred approach.

wpilibc/athena/src/SerialPort.cpp

@@ -134,6 +134,21 @@ int SerialPort::Read(char* buffer, int count) {
   return retVal;
 }
 
+/**
+ * Write raw bytes to the buffer. Deprecated, please use StringRef overload. Use
+ * Write({data, len}) to get a buffer that is shorter then the length of the
+ * std::string.
+ *
+ * @param buffer Pointer to the buffer to read the bytes from. If string.size()
+ * is less then count, only the length of string.size() will be sent.
+ * @param count  The maximum number of bytes to write.
+ * @return The number of bytes actually written into the port.
+ */
+int SerialPort::Write(const std::string& buffer, int count) {
+  return Write(llvm::StringRef(
+      buffer.data(), std::min(static_cast<int>(buffer.size()), count)));
+}
+
 /**
  * Write raw bytes to the buffer.
  *
@@ -141,10 +156,20 @@ int SerialPort::Read(char* buffer, int count) {
  * @param count  The maximum number of bytes to write.
  * @return The number of bytes actually written into the port.
  */
-int SerialPort::Write(const std::string& buffer, int count) {
+int SerialPort::Write(const char* buffer, int count) {
+  return Write(llvm::StringRef(buffer, static_cast<size_t>(count)));
+}
+
+/**
+ * Write raw bytes to the buffer.
+ *
+ * @param buffer StringRef to the buffer to read the bytes from.
+ * @return The number of bytes actually written into the port.
+ */
+int SerialPort::Write(llvm::StringRef buffer) {
   int32_t status = 0;
   int retVal = HAL_WriteSerial(static_cast<HAL_SerialPort>(m_port),
-                               buffer.c_str(), count, &status);
+                               buffer.data(), buffer.size(), &status);
   wpi_setErrorWithContext(status, HAL_GetErrorMessage(status));
   return retVal;
 }