Mark generated functions as #[inline]

[Amanieu]

This is needed to allow the code to be inlined across crates. This is part of my work to improve the performance of SymIterator in goblin.

[m4b]

So looking at other derives like Default, and Eq, these all also annotate the implementation with inline, so I'm ok with do it here, as the user can explicitly implement if they don't want this behavior.

Thanks for the PR!