Remove `register` keyword from public headers

Remove `register` keyword from public headers

In C++17 `register` keyword was removed. Current gcc 8.1.0 produces
following warning if `-std=c++17` flag is used:

warning: ISO C++17 does not allow 'register' storage class specifier
[-Wregister]

GCC almost completely ignores `register` keyword, with rare exception of
`-O0` when additional copy from/to stack may be generated.

For simplicity of the codebase it is better to just remove this
problematic keyword where it is not strictly required.

See: http://en.cppreference.com/w/cpp/language/storage_duration

Closes issue #345