fileinfo: implement os_fileinfo_size

this replaces os_get_file_size and file_info.stat.st_size

test/unit/os/fs_spec.lua

@@ -597,6 +597,20 @@ describe('fs function', function()
       end)
     end)
 
+    describe('os_fileinfo_size', function()
+      it('returns the correct size of a file', function()
+        local path = 'unit-test-directory/test.file'
+        local file = io.open(path, 'w')
+        file:write('some bytes to get filesize != 0')
+        file:flush()
+        file:close()
+        local size = lfs.attributes(path, 'size')
+        local file_info = file_info_new()
+        assert.is_true(fs.os_get_file_info(path, file_info))
+        eq(size, fs.os_fileinfo_size(file_info))
+      end)
+    end)
+
     describe('os_get_file_id', function()
       it('returns false if given an non-existing file', function()
         local file_id = file_id_new()