Add Support for Literal Types

[seandstewart]

Bugfixes:

• Lazy reprs eval to False, so we should check for a None when deciding on the error message at Constraints validation failure.
• Optional fields for recursive types were not properly reflected at Schema generation.

Features:

• Adds validation and deserialization for Literal types (resolves Validation of Literal types #87)

[codecov]

Codecov Report

Merging #129 into v2.1 will increase coverage by 0.05%.
The diff coverage is 93.03%.

@@            Coverage Diff             @@
##             v2.1     #129      +/-   ##
==========================================
+ Coverage   96.93%   96.99%   +0.05%     
==========================================
  Files          36       36              
  Lines        3854     3958     +104     
==========================================
+ Hits         3736     3839     +103     
- Misses        118      119       +1     

Flag	Coverage Δ
#3.8	96.99% <93.03%> (+0.05%)	⬆️
#unittests	96.99% <93.03%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
typic/constraints/__init__.py	100.00% <ø> (ø)
typic/klass.py	95.12% <ø> (ø)
typic/types/email.py	100.00% <ø> (ø)
typic/serde/translator.py	97.52% <80.00%> (-0.77%)	⬇️
typic/constraints/common.py	92.50% <88.88%> (-0.33%)	⬇️
typic/ext/schema/schema.py	97.23% <92.06%> (-0.12%)	⬇️
typic/api.py	96.88% <100.00%> (ø)
typic/checks.py	96.74% <100.00%> (+0.08%)	⬆️
typic/compat.py	100.00% <100.00%> (ø)
typic/constraints/factory.py	100.00% <100.00%> (ø)
... and 8 more

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[syastrov]

Very cool! 👍