This is a follow-up for #968

As discussed in #968, it would be nice to inline _isBaseCellPentagon for use in isValidCell (and potentially other spots in the code base).

I couldn't get this working in #968 after trying a mixture of inline and extern, so making this ticket to follow up and revisit.

Some notes:

• maybe benchmark the array of structs vs stuct of arrays in baseCellData