Store primitive GC arrays as raw byte buffers

[brendandahl]

GCData previously represented all allocations using a vector of Literals. Storing numeric array elements this way introduces unnecessary memory overhead and prevents efficient byte-level operations.

Represent primitive numeric GC arrays using a raw byte buffer in GCData while preserving Literals storage for reference arrays and structs.

[brendandahl]

This PR ends up causing quite a bit of churn, but it makes implementing multibyte load/stores from numeric arrays pretty easy compared to my original PR. This PR should also use less memory for numeric arrays and be faster for most operations on them. In some follow up PRs, we could also move strings into the vectors to make them more efficient.

I also explored a few other ways of implementing this:

1. Put all GC values in a byte array and do our own layout. This is probably the biggest change, and it seems relatively complex to handle all the alignment. Probably closer to what most runtimes do and would be the most efficient though.
2. Subclass GCData for literal and numeric arrays. Less churn but we'll then have virtual dispatch overhead.
3. Use two separate fields in GCData for Literals and bytes. This looks very similar to this PR, but seems a bit more error prone.

If anyone thinks we should just land my other PR or has other thoughts on how to do this let me know.

[tlively]

Is this an optimization, or would the getElement/setElement path below not do the right thing?

[brendandahl]

This is an optimization. I added a comment. Alternatively, I can remove and do optimizations in a follow up.