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)

test/filesystem_for_testing.h

@@ -50,7 +50,7 @@ class FakeFilesystem {
   FakeFile* FindFileOrNull(const std::string& filename) const;
 
  private:
-  size_t next_file_descriptor_ = 0;
+  int next_file_descriptor_ = 0;
   std::unordered_map<std::string, std::unique_ptr<FakeFile>> files_;
 };