[feat] Allow multiple marshals.

[lukesneeringer]

This commit removes the "marshal as singleton" component, allowing
different marshals to exist in tandem. In general, each package
should likely use its own marshal.

[codecov-io]

Codecov Report

Merging #9 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #9   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     13    +1     
  Lines         537    553   +16     
  Branches       88     90    +2     
=====================================
+ Hits          537    553   +16

Impacted Files	Coverage Δ
proto/marshal/rules/wrappers.py	100% <100%> (ø)
proto/marshal/rules/message.py	100% <100%> (ø)
proto/marshal/__init__.py	100% <100%> (ø)
proto/marshal/rules/dates.py	100% <100%> (ø)
proto/marshal/marshal.py	100% <100%> (ø)	⬆️
proto/__init__.py	100% <100%> (ø)	⬆️
proto/message.py	100% <100%> (ø)	⬆️

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