Unmarshal "allOf" with "oneOf/anyOf/allOf" sub-schemas

[sisp]

This PR adds support for unmarshaling allOf with nested sub-schemas of type oneOf/anyOf/allOf.

[codecov]

Codecov Report

Merging #44 (21ad0fe) into master (6c98890) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #44   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines          431       431           
  Branches        51        51           
=========================================
  Hits           431       431           

Impacted Files	Coverage Δ
src/oas/schema/unmarshalers.py	100.00% <100.00%> (ø)

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