Add C-style for loops

[yui-915]

"This is an extension, aiming to make using the language more convenient", by making it easier to iterate over range of numbers, example:

for (auto i = 0; i < 10; i += 2) printf("%lld\n", i);

instead of

auto i; i = 0; while (i < 10) {
    printf("%lld\n", i);
    i += 2;
}

This PR borrows syntax from #329 for the init-clause.