Remove the `length` fast path. A `length` read on a value typed unknown
that the caller checks as a number goes back to the `length` property
helper the class dispatch generates, and scr_dyn_array_length is removed.
The numeric index path (scr_dyn_index_get) is unchanged, and
IrFunction.fastRead now has only the `index` value.
Chris Tate
ad9934ebe1