Avoid "schemes" HashMap lookups during struct reads/writes

Generated Java code for structs doesn't need to do a HashMap lookup on every read/write operation.

Was surprised to see this show up in profilers under certain high throughput, write-heavy loads. Unclear if the cost was in the hashCode() implementation of the key or in the map lookup itself, but given this map never changes, for now it seems like it can be pretty safely replaced with a simple (and, in most cases, predictable) branch.