Fix a few compile warnings (#84)

Visual C++ reported a number of warnings:

..\src\filesystem.c(27): warning C4996: '_open': This function or variable may be unsafe. Consider using _sopen_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details.
..\src\filesystem.c(34): warning C4267: 'function': conversion from 'size_t' to 'unsigned int', possible loss of data
..\src\string_view.c(25): warning C4244: 'return': conversion from '__int64' to 'int', possible loss of data
..\src\string_view.c(41): warning C4244: 'return': conversion from '__int64' to 'int', possible loss of data
..\src\utils\list_cpu_features.c(151): warning C4090: 'function': different 'const' qualifiers
..\src\cpuinfo_x86.c(67): warning C4244: 'return': conversion from 'unsigned __int64' to 'uint32_t', possible loss of data
..\test\filesystem_for_testing.cc(48): warning C4267: 'return': conversion from 'size_t' to 'int', possible loss of data
..\test\filesystem_for_testing.cc(57): warning C4267: 'argument': conversion from 'size_t' to 'int', possible loss of data
..\test\bit_utils_test.cc(26): warning C4267: 'argument': conversion from 'size_t' to 'uint32_t', possible loss of data
..\test\bit_utils_test.cc(32): warning C4267: 'argument': conversion from 'size_t' to 'uint32_t', possible loss of data
..\test\bit_utils_test.cc(37): warning C4267: 'argument': conversion from 'size_t' to 'uint32_t', possible loss of data

Most of them, with the exception of _open() -> _sopen_s() are about
questionable implicit casts. Fixing the types and adding explicit casts
to explicitly mark the intention (and silence the warnings in the process)

src/cpuinfo_x86.c

@@ -64,7 +64,7 @@ Leaf CpuIdEx(uint32_t leaf_id, int ecx) {
   return leaf;
 }
 
-uint32_t GetXCR0Eax(void) { return _xgetbv(0); }
+uint32_t GetXCR0Eax(void) { return (uint32_t)_xgetbv(0); }
 
 #else
 #error "Unsupported compiler, x86 cpuid requires either GCC, Clang or MSVC."