Method invocations

Method invocations can have an implicit or explicit receiver and can have arguments in two forms:

 * Separated by parenthesis, and comma-delimited.

  ```
  method(12, 34)
  self.method(56, 78)
  ```
 * Space-delimited, to support nice-looking class and method declarations without adding syntax.

  ```
  puts 123 456
  self.puts 789 123
  ```