Multi dimensional array

[kchanqvq]

This PR implement support for multidimensional array, and :initial-contents argument to make-array.

Array representation is somewhat standardized, some details are changed:

1. We used to have arrays with no dimensions property floating around, and this leads to bugs like vector dimensions are wrong #488. Even after patches here and there, there's still missing cases. For this PR I first tried to plug all the holes -- which almost works, except for a. this creates lots of friction when using JavaScript functions that treat only vectors as first-class citizen; b. I realize JavaScript arrays passed from FFI would still be missing dimensions property anyways. So I settled on the solution in this PR: store dimensions property only for 0-D and multi-dimensional array, and vectors don't have dimensions property.
2. I removed type property. CL standard and other implementations don't support arbitrary array element types, and these are always tied to the underlying array representation. For our case stringp already has enough information. In the future we might want to support more element types by using JavaScript Typed Arrays.

BTW, how much do we care about legacy browsers? JavaScript Typed Arrays are ECMAScript 6, are we ok with requiring it? Or using it but provide a fallback?

p.s. Now that aref aset handle general multi-dimensional array, it's expected to have some performance penalty. I measured below 5% for tests.html. I think this is acceptable because I'd like to reach complete CL first. And the overhead can be easily removed once we have compiler macros.

[davazp]

BTW, how much do we care about legacy browsers? JavaScript Typed Arrays are ECMAScript 6, are we ok with requiring it? Or using it but provide a fallback?

I think TypedArray are fine. They would only be used in :element-type is specified right? so if we don't someone do not want to use them they can skip element-type.