Store a usd layer as a binary blob on the USD procedural

Add a `filedata` or similarly named byte array parameter to the usd procedural, so that we can reference a usd layer stored in the arnold scene instead of going to disk. See htoa ticket 1382.

Note the files may be as simple as

```
#usda 1.0
(
subLayers = [
@/data/sphere.usd@
]
)
```

and so even if multiple usd procedurals reference the same sub-layer, the USD stage should share these referenced usd files in memory.

Start at the USD procedural implementation and review htoa ticket 1382. Trace how a USD layer is loaded from disk, then determine how a byte-array parameter and shared in-memory sublayers should work. No files or tests are named; done means a scene can store and reference the layer without disk access while sharing referenced sublayers.

Written by the indexing model from the issue text.