CompactProtocol: more improvements

CompactProtocol: more improvements

Summary:
Add a specialized Cursor method that reads exactly 1 byte, in
addition to the existing one that reads into a buffer. Use it when
reading varints and standalone bytes in a CompactProtocol struct.

Test Plan:
```
------ old ------   ------ new ------
Benchmark                               time/iter  iters/s  time/iter  iters/s        win
=========================================================================================
CompactProtocol_read_Empty                18.68ns   53.54M    13.21ns   75.69M     41.37%
CompactProtocol_read_SmallInt             42.60ns   23.47M    29.95ns   33.39M     42.27%
CompactProtocol_read_BigInt               83.62ns   11.96M    68.40ns   14.62M     22.24%
CompactProtocol_read_SmallString          67.33ns   14.85M    55.62ns   17.98M     21.08%
CompactProtocol_read_BigString           353.83ns    2.83M   330.19ns    3.03M      7.07%
CompactProtocol_read_BigBinary           190.82ns    5.24M   182.90ns    5.47M      4.39%
CompactProtocol_read_LargeBinary         200.95ns    4.98M   187.00ns    5.35M      7.43%
CompactProtocol_read_Mixed               137.42ns    7.28M   102.98ns    9.71M     33.38%
CompactProtocol_read_SmallListInt        203.98ns    4.90M   146.68ns    6.82M     39.18%
CompactProtocol_read_BigListInt          120.50us    8.30K    71.56us   13.97K     68.31%
CompactProtocol_read_BigListMixed          1.62ms   617.07     1.26ms   795.60     28.93%
CompactProtocol_read_LargeListMixed      177.50ms     5.63   140.73ms     7.11     26.29%
```

Reviewed By: haijunz@fb.com

Subscribers: trunkagent, alandau, bmatheny, njormrod, mshneer, folly-diffs@

FB internal diff: D1678077

Signature: t1:1678077:1415923409:22accee6b62b6e2bf471f3758a290f71978a8c4e