thread safety through using task-local storage on non-inplace

[chriselrod]

functions; (sc::SimpleChain)(args...) and valgrad(...) w/out bang

Users should avoid using task_local_storage, especially in short lived tasks. If you must use it, try and have long-lived tasks, and communicate with them using channels.

But, probably worth defaulting to correctness for the sake of end users.

These are also the methods that are going to be hit in ForwardDiff and Zygote code that is likely to be multithreaded.

[chriselrod]

Note that this is actually currently less safe in one respect:
If you have multiple SimpleChains on the sake task, they'll use the same storage.

[codecov]

Codecov Report

Merging #86 (1293d93) into main (80696ec) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##             main      #86   +/-   ##
=======================================
  Coverage   71.37%   71.37%           
=======================================
  Files          13       13           
  Lines        2309     2313    +4     
=======================================
+ Hits         1648     1651    +3     
- Misses        661      662    +1     

Impacted Files	Coverage Δ
src/chain_rules.jl	20.37% <40.00%> (+1.13%)	⬆️
src/simple_chain.jl	87.02% <100.00%> (+0.14%)	⬆️

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