improve the tyencode abbrev format

improve the tyencode abbrev format

3% win on libcore

528828 liballoc-bb943c5a.rlib
1425126 liballoc_jemalloc-bb943c5a.rlib
10090 liballoc_system-bb943c5a.rlib
144904 libarena-bb943c5a.rlib
3773896 libcollections-bb943c5a.rlib
17075242 libcore-bb943c5a.rlib
195770 libflate-bb943c5a.rlib
234702 libfmt_macros-bb943c5a.rlib
536342 libgetopts-bb943c5a.rlib
212028 libgraphviz-bb943c5a.rlib
397068 liblibc-bb943c5a.rlib
185038 liblog-bb943c5a.rlib
680782 librand-bb943c5a.rlib
577574 librbml-bb943c5a.rlib
1381992 librustc_back-bb943c5a.rlib
37554736 librustc-bb943c5a.rlib
12826 librustc_bitflags-bb943c5a.rlib
2257392 librustc_borrowck-bb943c5a.rlib
533858 librustc_data_structures-bb943c5a.rlib
9338878 librustc_driver-bb943c5a.rlib
8960016 librustc_front-bb943c5a.rlib
1594212 librustc_lint-bb943c5a.rlib
79159342 librustc_llvm-bb943c5a.rlib
4590656 librustc_mir-bb943c5a.rlib
3529292 librustc_platform_intrinsics-bb943c5a.rlib
590688 librustc_privacy-bb943c5a.rlib
3084134 librustc_resolve-bb943c5a.rlib
14032890 librustc_trans-bb943c5a.rlib
11833852 librustc_typeck-bb943c5a.rlib
1641496 librustc_unicode-bb943c5a.rlib
15611582 librustdoc-bb943c5a.rlib
2693764 libserialize-bb943c5a.rlib
8266920 libstd-bb943c5a.rlib
29573790 libsyntax-bb943c5a.rlib
895484 libterm-bb943c5a.rlib