Remove register keyword from tokenizer extension
Giving the extension modules a look while working on my C knowledge I noticed that there were quite a few uses of the `register` keyword in tokenizer.c

I think it's worth removing this for the following reasons:

 - Python removed it on their end over 6 years ago back in 3.4 (see [here](https://bugs.python.org/issue18722))
 - Its usage (at least in gcc scope) is most likely considered a historical artifact and it was removed in C++17 (see [here](http://gcc.gnu.org/ml/gcc/2010-05/msg00098.html) and [here](http://clang.llvm.org/cxx_status.html#cxx17))
 - Its almost assuredly being ignored internally, because our usage of it in some places is incorrect; our code takes the address of variables declared with `register` which should be disallowed (see [here](https://github.com/pandas-dev/pandas/blob/7fafb356549f1c52a9896429ce0241487f42bea9/pandas/_libs/src/parser/tokenizer.c#L138))

Tried it out locally and saw no performance regression in removing. Will push a PR shortly

(edit: point 3 above might not be true; `->` has a higher precedence than `&` in C so might be OK in current code)