Add @inline to Base.reinterpret

[jakobnissen]

Reinterpret contains quite a bit of code to handle padding, and so doesn't usually inline for structs. However, it frequently compiles down to a noop, making an inline unusually valuable.

[jakobnissen]

One consideration is that reinterpret does expand to a fair bit of code if !Base.struct_subpadding(T, T2), and this change also inlines that case. One possibility is to refactor _reinterpret so that the noop case and the more complex case are two distinct functions, and only inline the latter.

[jakobnissen]

Test failures are unrelated