bug: Panic when `[` is not closed.

When I tried to assign an array to a variable, I missed to close parenthesis and then ion panicked. I tried to reproduce this behavior and found that using [ without ] after let foo = cause panic. If I forget to close other parenthesis, ion does not panic but only [ just after let foo = cause panic. I append examples to reproduce the error.

let foo = ( // => syntax error
let foo = { // => syntax error
let foo = [ // => panic!

( // => syntax error
{ // => syntax error
[ // => syntax error

let boo = (1, 2,  // => syntax error
let boo = {1, 2,  // => syntax error
let boo = [1, 2,  // => panic!