Reduce allocations when reusing a Builder.

Add the function `Reset` to the Builder, which facilitates reuse of the
underlying byte slice.

tests/GoTest.sh

@@ -43,7 +43,7 @@ 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=Build \
+                     --test.bench=. \
                      --test.benchtime=3s \
                      --fuzz=true \
                      --fuzz_fields=4 \