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()

examples/write_to_file.v

@@ -0,0 +1,26 @@
+import os
+import structlog
+
+fn main() {
+	// Open a file in append mode. If file does not exists it will be created.
+	log_path := os.join_path_single(os.temp_dir(), 'example_log')
+	log_file := os.open_file(log_path, 'a+') or {
+		eprintln('Error: cound not open log file ${log_path}: ${err}')
+		exit(1)
+	}
+
+	eprintln('Log file location: ${log_path}')
+
+	// Initialize logger with os.File as writer.
+	log := structlog.new(
+		handler: structlog.TextHandler{
+			color:  false
+			writer: log_file
+		}
+	)
+	defer {
+		log.close()
+	}
+
+	log.info().message('Hello, World!').send()
+}