Rework Buffer block

[User344]

This PR reworks current Buffer block. It adds another input to act as a trigger to output already stored messages received in main input port.

Here is the simplest example that would buffer a string and then output it each time ReadStdin block outputs anything (on new line).

use protoflow_blocks::*;
fn main() {
	System::run(|s| {
		let stdin = s.read_stdin();
		let hello = s.const_string("Hello, World!");
		let encode = s.encode_lines();
		let buffer = s.buffer();
		let stdout = s.write_stdout();
		s.connect(&hello.output, &encode.input);
		s.connect(&encode.output, &buffer.input);
		s.connect(&stdin.output, &buffer.trigger);
		s.connect(&buffer.output, &stdout.input);
	});
}

I am not sure what to do with StdioSystem implementation so for now I left it as it was before, meaning it just eats input and outputs nothing.