Feature: Support "in" keyword

[zauberparacelsus]

Python features an "in" keyword, which acts as quick syntax for checking to see if a particular element is contained within a list or other sequence.

Examples:

if(1 in [1, 2, 3]) {
    println("1 is in list")
}

if("a" in "abcdefg") {
    println("a is in string")
}

[danielperano]

This is a worthwhile feature - the "in" operator is only useable in for loops right now, but using it as a boolean test is unambiguous to the parser and fairly unambiguous to a human as well.