all: various improvements

* Add os.File.flush() call to fix writing log into files.
* Add comma as TextHandler fields separator.
* Add struct_adapter()
* Add field() generic function to creating Field instances.
* Add new write_to_file.v example.
* Rename Record.field() to Record.add()

.gitignore

@@ -22,3 +22,5 @@ bin/
 # vweb and database
 *.db
 *.js
+
+*.bak