L1VM - immutable

I did add two new compiler flags:
(variable-immutable)
(variable-mutable)
If the first is set then all following variables defined by set are immutable by default. To switch of this you need to use the second one.

Here is a small demo program (hello-mut.l1com): 

// Brackets - Hello world!
// set variables default as immutable
// change setting by: (variable-mutable)
//
#include <intr.l1h>
(main func)
    // all default variables are immutable after this is set!
    (variable-immutable)
    // immutable:
    (set int64 1 zero 0)
    (set int64 1 x 23)
    (set int64 1 y 42)
    // mutable:
    (set mut-int64 1 a 0)
    // string always mutable!
    (set string 13 hello "Hello world!")
    // print string
    print_s (hello)
    print_n
    ((x y *) a =)
    print_i (a)
    print_n
    exit (zero)
(funcend)