It's better than guessing based on the pointer size if the
implementation provides it directly.

There is no guarantee that Integer#size will continue to return
`sizeof(long)` for small integers.

Use the `l!` specifier for Array#pack instead. It is a public
interface that has a direct relationship with the `long` type.

What a "word" is when talking about sizes is confusing because it's a
highly overloaded term. Intel, Microsoft, and GDB are just a few vendors
that have their own definition of what a "word" is. Specs that used the
"wordsize" guard actually were mostly testing for the size of the C
`long` fundamental type, so rename the guard for clarity.

Also, get the size of `long` directly from RbConfig instead of guessing
using Integer#size. Integer#size is not guaranteed to have anything to
do with the `long` type.