add `@inline` annotations to `[getindex|setindex!](::ACSet, ...)`

[aviatesk]

These functions can be significantly optimized with constant information available within their arguments, so it would be beneficial to enforce constant propagation for them. This can be achieved using Base.@constprop :aggressive or @inline, and since these functions are likely to be used frequently, I think promoting inline expansion with @inline is a good approach, so I chose to use @inline.

• fixes #121

[jpfairbanks]

@olynch it looks like the Java integration is holding up the tests on OS X. Is there any reason to believe that these changes are affecting that error?

[epatters]

CI issue fixed in #134. I guess arm64 is now the most stable Mac architecture!