Handle mapping struct fields with subtypes to Objective-C

[julianlocke]

Addresses dropbox/SwiftyDropbox#399. See dropbox/SwiftyDropbox#400 for change in generated code. This code checks to see if a wrapped field type has user defined subclasses, and if it does, switches over them to ensure we instantiate the right one.

Checklist

General Contributing

• Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• Code Change
• Example/Test Code Change

Validation

• Have you ran tox?
• Do the tests pass?

[codecov]

Codecov Report

Attention: 84 lines in your changes are missing coverage. Please review.

Comparison is base (94fa542) 50.30% compared to head (54c75f0) 49.96%.
Report is 11 commits behind head on swifty-dropbox-beta.

❗ Current head 54c75f0 differs from pull request most recent head 8dfd93e. Consider uploading reports for the commit 8dfd93e to get more accurate results

Files	Patch %	Lines
stone/backends/swift_types.py	0.00%	45 Missing ⚠️
stone/backends/swift_client.py	0.00%	22 Missing ⚠️
stone/cli.py	0.00%	16 Missing ⚠️
stone/frontend/ir_generator.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           swifty-dropbox-beta     #319      +/-   ##
=======================================================
- Coverage                50.30%   49.96%   -0.35%     
=======================================================
  Files                       37       37              
  Lines                     8740     8800      +60     
  Branches                  1881     1887       +6     
=======================================================
  Hits                      4397     4397              
- Misses                    4026     4086      +60     
  Partials                   317      317              

Flag	Coverage Δ
unit	49.96% <1.17%> (-0.35%)	⬇️

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[jaquish]

LGTM

Pairs with dropbox/SwiftyDropbox#400