Remove submodule overhead if no submodules

[joshua-spacetime]

Description of Changes

Before this change, every reducer call did the following:

1. Checked whether it belonged to the root module or a submodule.
2. Retrieved the appropriate ctx.db.
3. Retrieved the appropriate ctx.as.
4. Reassigned both properties on the reusable reducer context.

This was true whether or not the module had any submodules, and this added meaningful overhead especially for high throughput workloads, such as the keynote benchmark.

Now if a module doesn't have any submodules, reducer calls take an early fast path. It directly selects the root reducer and resets only values that genuinely change each transaction.

API and ABI breaking changes

N/A

Expected complexity level and risk

1

Testing

• Revert keynote bench throughput

[joshua-spacetime]

So far, none of these changes have had any impact on the keynote bench throughput.