Fix at+get fns

[goastler]

• finite index check (because nan and inf suck)
• generic array inference fix (ts infers the array correctly so you don't have to)
• undefined case handling fix (make sure an array containing an undefined cannot return Exclude<X, undefined>)
• fix string calling of at fn (wouldn't always pickup string correctly)
• fix return types with/wo undefined excluded
• accept readonly arrays in at fn