fix issues with MSVC

[h-vetinari]

While working on conda-forge/s2geometry-feedstock#35, I ran into three separate issues with MSVC which are all fixed here.

Problem 1: constexpr

%SRC_DIR%\src\s2\util\math\exactfloat\bignum.cc(96): error C3493: 'kMaxChunkDigits' cannot be implicitly captured because no default capture mode has been specified

fixed by making kMaxChunkDigits be of static storage duration

Problem 2: namespace confusion between C(++) stdlib vs. ExactFloat implementations

│ │ FAILED: [code=2] CMakeFiles/s2.dir/src/s2/util/math/exactfloat/exactfloat.cc.obj
│ │ C:\PROGRA~1\MICROS~2\2022\ENTERP~1\VC\Tools\MSVC\1444~1.352\bin\Hostx64\x64\cl.exe /nologo /TP -DABSL_CONSUME_DLL -DABSL_MIN_LOG_LEVEL=1 -DNOMINMAX -DS2GEOMETRY_BUILDING_S2GEOMETRY -D_USE_MATH_DEFINES -Ds2_EXPORTS -I%SRC_DIR%\src -external:I%PREFIX%\Library\include -external:W0 /DWIN32 /D_WINDOWS /GR /EHsc /O2 /Ob2 /DNDEBUG -std:c++17 -MD -J /showIncludes /FoCMakeFiles\s2.dir\src\s2\util\math\exactfloat\exactfloat.cc.obj /FdCMakeFiles\s2.dir\ /FS -c %SRC_DIR%\src\s2\util\math\exactfloat\exactfloat.cc
│ │ %SRC_DIR%\src\s2\util\math\exactfloat\exactfloat.cc(59): error C2440: 'initializing': cannot convert from 'exactfloat::ExactFloat' to 'double'
│ │ %SRC_DIR%\src\s2\util\math\exactfloat\exactfloat.cc(59): note: No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
│ │ %SRC_DIR%\src\s2\util\math\exactfloat\exactfloat.cc(60): error C2440: 'static_cast': cannot convert from 'exactfloat::ExactFloat' to 'uint64_t'
│ │ %SRC_DIR%\src\s2\util\math\exactfloat\exactfloat.cc(60): note: No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
│ │ %SRC_DIR%\src\s2\util\math\exactfloat\exactfloat.cc(149): error C2440: 'return': cannot convert from 'exactfloat::ExactFloat' to 'double'
│ │ %SRC_DIR%\src\s2\util\math\exactfloat\exactfloat.cc(149): note: No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

Fixed by qualifying the respective calls with std:: (since <cmath> is already included)

Problem 3: std::from_chars for integers takes pointers, not iterators

 │ │ FAILED: [code=2] CMakeFiles/s2.dir/src/s2/util/math/exactfloat/bignum.cc.obj 
 │ │ C:\PROGRA~1\MICROS~2\2022\ENTERP~1\VC\Tools\MSVC\1444~1.352\bin\Hostx64\x64\cl.exe  /nologo /TP -DABSL_CONSUME_DLL -DABSL_MIN_LOG_LEVEL=1 -DNOMINMAX -DS2GEOMETRY_BUILDING_S2GEOMETRY -D_USE_MATH_DEFINES -Ds2_EXPORTS -I%SRC_DIR%\src -external:I%PREFIX%\Library\include -external:W0 /DWIN32 /D_WINDOWS /GR /EHsc /O2 /Ob2 /DNDEBUG -std:c++17 -MD   -J /showIncludes /FoCMakeFiles\s2.dir\src\s2\util\math\exactfloat\bignum.cc.obj /FdCMakeFiles\s2.dir\ /FS -c %SRC_DIR%\src\s2\util\math\exactfloat\bignum.cc
 │ │ %SRC_DIR%\src\s2\util\math\exactfloat\bignum.cc(124): error C2665: 'std::from_chars': no overloaded function could convert all the argument types
 │ │ C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\charconv(1960): note: could be 'std::from_chars_result std::from_chars(const char *const ,const char *const ,long double &,const std::chars_format) noexcept'
 │ │ %SRC_DIR%\src\s2\util\math\exactfloat\bignum.cc(124): note: 'std::from_chars_result std::from_chars(const char *const ,const char *const ,long double &,const std::chars_format) noexcept': cannot convert argument 1 from 'std::_String_view_iterator<_Traits>' to 'const char *const '
 │ │         with
 │ │         [
 │ │             _Traits=std::char_traits<char>
 │ │         ]

Fixed by using pointers.