- `GlobalMetadata` - offsets for global kernel variables in memory
- `ContextMetadata` - offsets for context-specific kernel variables in memory
- `GAS_CONSTANTS`, based on the yellowpaper
Also move constants to a separate module since `aggregator` was getting long.
It's a bit more type-safe (can't mix up segment with context or virtual addr), and this way uniqueness of ordinals is enforced, partially addressing a concern raised in #591.
To avoid making `Segment` public (which I don't think would be appropriate), I had to make some other visibility changes, and had to move `generate_random_memory_ops` into the test module.
