User actions

[BastianBlokland]

This pr adds user defined actions, meaning functions with side-effecs. Actions can call other actions and function but functions can only call other functions.

This also adds a few more build-in actions:

print(char)
printl(string) // Print line: like normal print(string) but outputs a newline after the string.
read() // Read: Returns a character from stdin.
readl() // Read line: Returns a string from stdin.
sleep(int) // Sleep for x milliseconds.

Example:

action greetEveryone()
  printl("Who are you?");
  name = readl();
  if name.length() == 0 -> 0
  else                  -> printl("Hi: " + name); sleep(1000); greetEveryone()

greetEveryone()