Simplified istream handing #367

Error : 'identifier "size_t" is undefined' in linux

Fix Visual Studio 2017 warnings

Fix warning C4706 on Visual Studio 2017 - fixes #784

Set GENERATE_TAGFILE in Doxyfile

Lots of tests have this warning.

Also moved out of for loop, doesn't need to be done every loop.

This causes test-udt to crash due to bad iterator

Allows documentation to be linked from other projects to https://nlohmann.github.io/json using Doxygen with the tag:
TAGFILES = $(SOME_PATH)/nlohmann_json.tag=https://nlohmann.github.io/json

o For some unknown reason, the complexity of the benchmark platform
  prevented some C++ compilers from generating optimal code, properly
  reflective of the real performance in actual deployment.
o Added the json_benchmarks_simple target, which performs the same
  suite of tests as json_benchmarks.
o Simplified the benchmark platform, and emit an "Average" TPS
  (Transactions Per Second) value reflective of aggregate parse/output
  performance.

o We assume the same character int_type as the unerlying std::istream
o There are no assumptions on the value of eof(), other than that it
  will not be a valid unsigned char value.
o To retain performance, we do not allow swapping out the underlying
  std::streambuf during our use of the std::istream for parsing.

o Make no assumptions about eof(), other than that it is somewhere
  outside of the valid range of char_type.

o We can retain -Weffc++ and specify default initializers by using
  initializer lists.  The risks are low (of additional non-conformat
  compilers), because there is already one other such initialization
  used in the code-base.

o An (-'ve valued, typically -1) EOF must never be allowed in
  token_string, as it be converted to 255 -- a legitimate value.
o Comparing against a specific eof() (-1, typically) is more costly than
  detecting +'ve/-'ve.  Since EOF is the only non-positive value allowed
  we can use the simpler test.
o Removed unnecessary test for token_string size, as it is already
  tested in the method, and must never occur in correct code; used an
  assert instead.

o Return its contents when necessary.  In many cases, this avoids
  construction of multiple copies of the yytext token.  Exceeds
  performance of current develop branch.

o Use std::streambuf I/O instead of std::istream; does not maintain
  (unused) istream flags.
o Further simplify get/unget handling.
o Restore original handling of NUL in input stream; ignored during
  token_string escaping.