Introduce named global executor for choice in execution

[kezhuw]

It is a good for binary crate to active at most one global spawner
compat. But it could be relatively hard if dependency graph is large.
In this case, it would be good to offer choices in execution.

This commit uses environment variable SPAWNS_GLOBAL_EXECUTOR to choose
one in absent of thread context spawners.

[codecov]

Codecov Report

Attention: Patch coverage is 96.92308% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 95.27%. Comparing base (3f38b2d) to head (63a7afb).

Files	Patch %	Lines
spawns-core/src/compat.rs	95.91%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
- Coverage   95.28%   95.27%   -0.02%     
==========================================
  Files           7        7              
  Lines         954      994      +40     
==========================================
+ Hits          909      947      +38     
- Misses         45       47       +2     

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