Variadic functions, Nil type and many bug fixes

Features

• Variadic functions in the ...args form.
• New Nil data type.

Improvements

• Short circuit boolean AND and OR.
• Open functions scope to allow calling other functions.
• Pipe operator supports any data type.
• Dictionary keys can be of any data type.
• Return nil on inexistent dictionary keys instead of runtime error.
• Return nil on inexistent array keys instead of runtime error.
• Empty or placeholder array index assignment appends a value to the array.
• Division by zero triggers a runtime error.
• Arrays or dictionaries can be compared with comparison operators.

Bug Fixes

• Assignment would update the current block scope, but not the parent.
• Multiple calls on a function would carry over the scope.
• Return in for in loop wouldn't cause the loop to stop and return a value immediately.