Parallel sections can be improved once RFC 2229 is resolved

[fmckeogh]

Tracking issue: rust-lang/rust#53488

"Closures will capture individual fields and not the whole struct when possible"

This prevents taking a mutable/immutable reference to any other field of State when an array is being used in a parallel section. This is resolved by either creating temporary variables and assigning to the fields after the closure has completed, or by not parallelising at all. Once this issue is closed however these sections can be improved.