Remove all string allocations during parsing.

Change the signature for 'string' getters and settings to use byte
slices instead of strings.

tests/GoTest.sh

@@ -43,6 +43,8 @@ GOPATH=${go_path} go test flatbuffers_test \
                      --test.coverpkg=github.com/google/flatbuffers/go \
                      --cpp_data=${test_dir}/monsterdata_test.mon \
                      --out_data=${test_dir}/monsterdata_go_wire.mon \
+                     --test.bench=. \
+                     --test.benchtime=5s \
                      --fuzz=true \
                      --fuzz_fields=4 \
                      --fuzz_objects=10000