Skip no-op adjustments in trans
That allows us to keep using trans_into() in case of adjustments that
may actually be ignored in trans because they are a plain deref/ref pair
with no overloaded deref or unsizing.
Unoptimized(!) benchmarks from servo/servo#7638
Before
```
test goser::bench_clone ... bench: 17,701 ns/iter (+/- 58) = 30 MB/s
test goser::bincode::bench_decoder ... bench: 33,715 ns/iter (+/- 300) = 11 MB/s
test goser::bincode::bench_deserialize ... bench: 36,804 ns/iter (+/- 329) = 9 MB/s
test goser::bincode::bench_encoder ... bench: 34,695 ns/iter (+/- 149) = 11 MB/s
test goser::bincode::bench_populate ... bench: 18,879 ns/iter (+/- 88)
test goser::bincode::bench_serialize ... bench: 31,668 ns/iter (+/- 156) = 11 MB/s
test goser::capnp::bench_deserialize ... bench: 2,049 ns/iter (+/- 87) = 218 MB/s
test goser::capnp::bench_deserialize_packed ... bench: 10,707 ns/iter (+/- 258) = 31 MB/s
test goser::capnp::bench_populate ... bench: 635 ns/iter (+/- 5)
test goser::capnp::bench_serialize ... bench: 35,657 ns/iter (+/- 155) = 12 MB/s
test goser::capnp::bench_serialize_packed ... bench: 37,881 ns/iter (+/- 146) = 8 MB/s
test goser::msgpack::bench_decoder ... bench: 50,634 ns/iter (+/- 307) = 5 MB/s
test goser::msgpack::bench_encoder ... bench: 25,738 ns/iter (+/- 90) = 11 MB/s
test goser::msgpack::bench_populate ... bench: 18,900 ns/iter (+/- 138)
test goser::protobuf::bench_decoder ... bench: 2,791 ns/iter (+/- 29) = 102 MB/s
test goser::protobuf::bench_encoder ... bench: 75,414 ns/iter (+/- 358) = 3 MB/s
test goser::protobuf::bench_populate ... bench: 19,248 ns/iter (+/- 92)
test goser::rustc_serialize_json::bench_decoder ... bench: 109,999 ns/iter (+/- 797) = 5 MB/s
test goser::rustc_serialize_json::bench_encoder ... bench: 58,777 ns/iter (+/- 418) = 10 MB/s
test goser::rustc_serialize_json::bench_populate ... bench: 18,887 ns/iter (+/- 76)
test goser::serde_json::bench_deserializer ... bench: 104,803 ns/iter (+/- 770) = 5 MB/s
test goser::serde_json::bench_populate ... bench: 18,890 ns/iter (+/- 69)
test goser::serde_json::bench_serializer ... bench: 75,046 ns/iter (+/- 435) = 8 MB/s
```
After
```
test goser::bench_clone ... bench: 16,052 ns/iter (+/- 188) = 34 MB/s
test goser::bincode::bench_decoder ... bench: 31,194 ns/iter (+/- 941) = 12 MB/s
test goser::bincode::bench_deserialize ... bench: 33,934 ns/iter (+/- 352) = 10 MB/s
test goser::bincode::bench_encoder ... bench: 30,737 ns/iter (+/- 1,969) = 13 MB/s
test goser::bincode::bench_populate ... bench: 17,234 ns/iter (+/- 176)
test goser::bincode::bench_serialize ... bench: 28,269 ns/iter (+/- 452) = 12 MB/s
test goser::capnp::bench_deserialize ... bench: 2,019 ns/iter (+/- 85) = 221 MB/s
test goser::capnp::bench_deserialize_packed ... bench: 10,662 ns/iter (+/- 527) = 31 MB/s
test goser::capnp::bench_populate ... bench: 607 ns/iter (+/- 2)
test goser::capnp::bench_serialize ... bench: 30,488 ns/iter (+/- 219) = 14 MB/s
test goser::capnp::bench_serialize_packed ... bench: 33,731 ns/iter (+/- 201) = 9 MB/s
test goser::msgpack::bench_decoder ... bench: 46,921 ns/iter (+/- 461) = 6 MB/s
test goser::msgpack::bench_encoder ... bench: 22,315 ns/iter (+/- 96) = 12 MB/s
test goser::msgpack::bench_populate ... bench: 17,268 ns/iter (+/- 73)
test goser::protobuf::bench_decoder ... bench: 2,658 ns/iter (+/- 44) = 107 MB/s
test goser::protobuf::bench_encoder ... bench: 71,024 ns/iter (+/- 359) = 4 MB/s
test goser::protobuf::bench_populate ... bench: 17,704 ns/iter (+/- 104)
test goser::rustc_serialize_json::bench_decoder ... bench: 107,867 ns/iter (+/- 759) = 5 MB/s
test goser::rustc_serialize_json::bench_encoder ... bench: 52,327 ns/iter (+/- 479) = 11 MB/s
test goser::rustc_serialize_json::bench_populate ... bench: 17,262 ns/iter (+/- 68)
test goser::serde_json::bench_deserializer ... bench: 99,156 ns/iter (+/- 657) = 6 MB/s
test goser::serde_json::bench_populate ... bench: 17,264 ns/iter (+/- 77)
test goser::serde_json::bench_serializer ... bench: 66,135 ns/iter (+/- 392) = 9 MB/s
```
2c17bfc20c