improve type stability and runtime performance

[ranocha]

This is part of an effort to improve the performance of some of our numerical simulation methods for partial differential equations in Trixi.jl. The changes in this PR improve the performance of our methods by ca. 25%. Thus, it would be really great if this could be merged and a new version of StructArrays.jl could be registered.

[piever]

Thanks for the PR! Did you happen to measure if the compilation overhead when doing using StructArrays after this change compared with master?

[ranocha]

Running julia -e '@time using StructArrays' with Julia 1.7.0-beta3.0 (2021-07-07) several times yields the following results.

• Current master: A typical result is 0.533961 seconds (2.59 M allocations: 182.671 MiB, 3.20% gc time, 0.88% compilation time); the times vary slightly by ca. 0.02 seconds
• This PR: A typical result is 0.536773 seconds (2.59 M allocations: 182.672 MiB, 3.23% gc time, 0.88% compilation time); the times vary slightly by ca. 0.02 seconds

Thus, the latency of using StructArrays did not really change.