Javascript: Prefer typed arrays when possible for repeat-expr
In javascript, when using `repeat-expr` for types like `u1`, `s1`, `u2`, `s2`, `u4`, `s4`, `f4`, `f8` etc, it'd be lovely if `ksc` used typed arrays (ie `Uint8Array`, `Float32Array`, etc) instead of generic `Array`s.

@cyco already started in on this here: https://github.com/cyco/kaitai_struct_compiler/commit/b3ce66bb0ab51400f2fa6665696b2cf52d4b2e03

Among other benefits: typed arrays can be transferred off of web workers [at essentially no cost](https://developer.mozilla.org/en-US/docs/Web/API/Transferable), making it convenient to run a parser in a web worker. In contrast, generic arrays must be copied when moving them between web workers and the main thread.