Support compiling with C++17/C++2a

[emfrias]

Bitshares used 'using namespace std' in several places, and this
caused conflicts between fc::variant and the new std::variant.
Fixed by just importing the specific identifiers we use from
namespace std.

[abitmore]

@emfrias please send pull request to develop branch.

[abitmore]

Please undo the change on FC submodule.

[pmconrad]

Looks good, except for the fc downgrade.

[pmconrad]

Thanks!
@jmjatlanta can you test this on windows?

[jmjatlanta]

@jmjatlanta can you test this on windows?

Tested on Windows 10 / Boost 1.69 / Visual Studio 2017 with no problems.

Note unrelated to this ticket: With the current status of fc and bitshares-core, to compile with Visual Studio 2017 you must

cd libraries/fc
git checkout master
cd ../../

And then edit the file libraries/chain/include/graphene/chain/protocol/types.hpp and remove the section between the //TODO comments near the top. Then compile as normal. For me that is:

cmake -G %VS_VERSION% -A x64 -DBOOST_ROOT=%BOOST_DIR% -DCURL_STATICLIB=ON -DCURL_LIBRARY=%CURL_DIR%\lib\libcurl_imp.lib -DCURL_INCLUDE_DIR=%CURL_DIR%\include -DOPENSSL_CONF_SOURCE="C:\Program Files\Common Files\SSL\openssl.cnf"
cmake --build . --target install --config Release

(replace the variables with values specific to your environment)

This should become easier as we approach the 3.2.0 release.