Array operations

[simonask]

Add the following functionality:

• Array index lookup syntax array[N] in expressions - consider whether N can also be a stringly-typed identifier referring to a variable that can be parsed as an integer.
• Array index lookup syntax in string interpolations: {array[N]}
• Chaining ops: first, last, len, head, tail
• Simple destructuring: let [a, b] = array
• Advanced destructuring: let [first, ..tail] = array and let [..head, last] = array