Inline r_pairlist_find()

[DavisVaughan]

r_pairlist_find() was showing up in my Instruments profiles as not being inlined. It is used in some of the deepest layers of vctrs via r_dim() and r_class() through the following:

• r_dim()
• calls r_attrib_get()
• calls r_pairlist_get()
• calls r_pairlist_find()

r_dim() itself is used on every iteration of vec_size_common() when computing the vec_size() of an object.

It's definitely a simple enough loop that we can inline it, and after inlining it does disappear from the Instruments profile