Avoid quadratic allocation when collecting reactions and equations from subsystems

[nifets]

Passing init to reduce(vcat, xs) in reactions(network) and MT.equations(::ReactionSystem)opts out of the optimised reduce dispatch, resulting in quadratic allocation when dealing with hiearchical models. It is also unnecessary, as both functions return early when systems is empty.

Example allocations (MiB) for a system with N subsystems of one reaction pair each:

N	reactions master	reactions this PR	equations master	equations this PR
1024	19.4	10.8	18.9	10.2
4096	189.4	43.3	187.7	40.9
8192	648.2	89.6	644.7	81.7

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

[isaacsas]

Thanks! I assume the QA test failures are unrelated, so we can merge once the doc build passes.

[ChrisRackauckas]

Yeah that's a blanket reexport thing. Let me change that up so we're not just reexporting everything. I think the tests look fine from this.