Fix "xtime: ambiguous symbol" with Visual Studio.

[Quipyowert2]

This pull request fixes the cause of an error I was getting when compiling PR #64 in Visual Studio. The problem is that there is both a xtime symbol (defined by Visual Studio in xtimec.h) and a boost::xtime symbol (which is aliased to xtime by using namespace boost). Replacing all the using namespace std; and using namespace boost; with a using statement specifying the exact type to use (e.g. using std::string;) fixes the error and reduces the possibility of similar compiler errors in the future.

The error I was receiving was the following, repeated several times for several different source files:

1>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.35.32215\include\condition_variable(143): error C2872: 'xtime': ambiguous symbol
1>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.35.32215\include\xtimec.h(23,8): message : could be 'xtime'
1>D:\Linux_home\nathan\src\glob2\vcpkg_installed\x64-windows\x64-windows\include\boost\thread\lock_types.hpp(31,10): message : or       'boost::xtime'
1>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.35.32215\include\condition_variable(148): error C2872: 'xtime': ambiguous symbol
1>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.35.32215\include\xtimec.h(23,8): message : could be 'xtime'
1>D:\Linux_home\nathan\src\glob2\vcpkg_installed\x64-windows\x64-windows\include\boost\thread\lock_types.hpp(31,10): message : or       'boost::xtime'
1>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.35.32215\include\condition_variable(258): error C2872: 'xtime': ambiguous symbol
1>C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.35.32215\include\xtimec.h(23,8): message : could be 'xtime'
1>D:\Linux_home\nathan\src\glob2\vcpkg_installed\x64-windows\x64-windows\include\boost\thread\lock_types.hpp(31,10): message : or       'boost::xtime'

build.log.

[Quipyowert2]

Oops, forgot to remove a few using namespace lines. The error is still there. Converting to draft for now. Probably going to work on this tomorrow.