Add a Void type

The fact that Mars lacks a Void type (a type with only a single value, meaning "no information") is annoying for two reasons: Firstly, functions which don't return anything need to be given an Int return type and always feature "return 0", and secondly, in the interactive mode, a 0 is always printed.

Note that you can write your own Void type (type Void: Void), but it wouldn't solve either problem because you would still need to write "return Void", and it would still print "Void".

I did not add a Void type for a long time because Mars is supposed to be a pure language, and it's not useful to have a pure function which returns no information. However, it has impure functions which perform IO (such as put_char), and those are fairly useful.

Add a new built-in type Void which has a single value, Void. Add two special rules. Functions with a Void return type do not require a return statement; they implicitly "return Void" at the end of the function body. In interactive mode, results of expressions of type Void are not printed. Note that Void is still a proper value; it can be passed as arguments or stored in variables (unlike in C), and an explicit "return Void" is also allowed. Also add a new rule that "return" without an expression implicitly returns Void (and only works in Void functions).

Note: If we implement subroutines, this would be useless (as all "Void" functions could just be subroutines without outputs). So consider implementing subroutines before going ahead with this.