Conform to C++ standard

[htfy96]

Hi,
this is an awesome repository which contains a lot of useful algorithms for C++, but the code style, I think, could be improved:

• Use <cstddef> instead of <stddef.h>
• each identifier with two underscores at the beginning(__foobar) or one underscore followed by character in uppercase(_Foobar) is reserved by language
• the void keyword could be omitted inint main(void), so is struct in struct Foo obj

[xtaci]

can you provide Pull Requests?

[htfy96]

#51