Remove stddef dependency

[Frayzen]

The repo claims to be dependy-free. Let's change size_t to unsigned long and remove the include.
All the tests are still working after modification.

[syoyo]

unsigned long on Windows 64bit is 32bit, so we'll need to define custom size_t(4 bytes on 32bit platform, 8 bytes on 64bit) like this:

https://stackoverflow.com/questions/68920256/whats-the-difference-with-size-t-and-unsigned-long