Add SaveEntries() and LoadEntries(). (#233)

These allow saving and loading non-persistent entries in the persistent
file format.

src/main/native/include/networktables/NetworkTableInstance.h

@@ -430,7 +430,7 @@ class NetworkTableInstance final {
   /** @} */
 
   /**
-   * @defgroup PersistentFunctions Persistent Functions
+   * @defgroup FileFunctions File Save/Load Functions
    * @{
    */
 
@@ -455,6 +455,27 @@ class NetworkTableInstance final {
       StringRef filename,
       std::function<void(size_t line, const char* msg)> warn);
 
+  /**
+   * Save table values to a file.  The file format used is identical to
+   * that used for SavePersistent.
+   * @param filename  filename
+   * @param prefix    save only keys starting with this prefix
+   * @return error string, or nullptr if successful
+   */
+  const char* SaveEntries(StringRef filename, StringRef prefix) const;
+
+  /**
+   * Load table values from a file.  The file format used is identical to
+   * that used for SavePersistent / LoadPersistent.
+   * @param filename  filename
+   * @param prefix    load only keys starting with this prefix
+   * @param warn      callback function for warnings
+   * @return error string, or nullptr if successful
+   */
+  const char* LoadEntries(
+      StringRef filename, StringRef prefix,
+      std::function<void(size_t line, const char* msg)> warn);
+
   /** @} */
 
   /**