Add perf tests for EncodedInputStream and AutoUTFInputStream

[spl]

I think we need more performance tests with the different variations of parsing. Here are two more.

For the curious, here are my results:

$ ./perftest_release_x64_gmake
[==========] Running 23 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 23 tests from RapidJson
[ RUN      ] RapidJson.ReaderParseInsitu_DummyHandler
[       OK ] RapidJson.ReaderParseInsitu_DummyHandler (836 ms)
[ RUN      ] RapidJson.ReaderParseInsitu_DummyHandler_ValidateEncoding
[       OK ] RapidJson.ReaderParseInsitu_DummyHandler_ValidateEncoding (1278 ms)
[ RUN      ] RapidJson.ReaderParse_DummyHandler
[       OK ] RapidJson.ReaderParse_DummyHandler (819 ms)
[ RUN      ] RapidJson.ReaderParseIterative_DummyHandler
[       OK ] RapidJson.ReaderParseIterative_DummyHandler (864 ms)
[ RUN      ] RapidJson.ReaderParseIterativeInsitu_DummyHandler
[       OK ] RapidJson.ReaderParseIterativeInsitu_DummyHandler (865 ms)
[ RUN      ] RapidJson.ReaderParse_DummyHandler_ValidateEncoding
[       OK ] RapidJson.ReaderParse_DummyHandler_ValidateEncoding (1417 ms)
[ RUN      ] RapidJson.DocumentParseInsitu_MemoryPoolAllocator
[       OK ] RapidJson.DocumentParseInsitu_MemoryPoolAllocator (873 ms)
[ RUN      ] RapidJson.DocumentParseIterativeInsitu_MemoryPoolAllocator
[       OK ] RapidJson.DocumentParseIterativeInsitu_MemoryPoolAllocator (910 ms)
[ RUN      ] RapidJson.DocumentParse_MemoryPoolAllocator
[       OK ] RapidJson.DocumentParse_MemoryPoolAllocator (901 ms)
[ RUN      ] RapidJson.DocumentParseIterative_MemoryPoolAllocator
[       OK ] RapidJson.DocumentParseIterative_MemoryPoolAllocator (920 ms)
[ RUN      ] RapidJson.DocumentParse_CrtAllocator
[       OK ] RapidJson.DocumentParse_CrtAllocator (1281 ms)
[ RUN      ] RapidJson.DocumentParseEncodedInputStream_MemoryStream
[       OK ] RapidJson.DocumentParseEncodedInputStream_MemoryStream (1856 ms)
[ RUN      ] RapidJson.DocumentParseAutoUTFInputStream_MemoryStream
[       OK ] RapidJson.DocumentParseAutoUTFInputStream_MemoryStream (2926 ms)
[ RUN      ] RapidJson.DocumentTraverse
[       OK ] RapidJson.DocumentTraverse (17 ms)
[ RUN      ] RapidJson.DocumentAccept
[       OK ] RapidJson.DocumentAccept (38 ms)
[ RUN      ] RapidJson.Writer_NullStream
[       OK ] RapidJson.Writer_NullStream (116 ms)
[ RUN      ] RapidJson.Writer_StringBuffer
[       OK ] RapidJson.Writer_StringBuffer (497 ms)
[ RUN      ] RapidJson.PrettyWriter_StringBuffer
[       OK ] RapidJson.PrettyWriter_StringBuffer (567 ms)
[ RUN      ] RapidJson.internal_Pow10
[       OK ] RapidJson.internal_Pow10 (2 ms)
[ RUN      ] RapidJson.Whitespace
[       OK ] RapidJson.Whitespace (664 ms)
[ RUN      ] RapidJson.UTF8_Validate
[       OK ] RapidJson.UTF8_Validate (2906 ms)
[ RUN      ] RapidJson.FileReadStream
[       OK ] RapidJson.FileReadStream (734 ms)
[ RUN      ] RapidJson.ReaderParse_DummyHandler_FileReadStream
[       OK ] RapidJson.ReaderParse_DummyHandler_FileReadStream (1683 ms)
[----------] 23 tests from RapidJson (22970 ms total)

[----------] Global test environment tear-down
[==========] 23 tests from 1 test case ran. (22970 ms total)
[  PASSED  ] 23 tests.