Adds reader for valid_values

[popematt]

Issue #, if available:

#256

Description of changes:

• Adds ValidValuesReader and tests
• Adds extension functions for converting from IonValue to NumberRange and TimestampRange.
• While working on this, I ran into problems with the BigDecimal and Timestamp both having inconsistent compareTo and equals, so I introduced ConsistentDecimal and ConsistentTimestamp as wrappers that properly implement equals(), compareTo() and hashCode() in a way that all three are consistent with each other.
• There is also an incidental change to ReaderTests that makes the test failure messages a little bit more helpful.

Related PRs in ion-schema, ion-schema-tests, ion-schema-schemas:

None

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Patch coverage: 84.61% and project coverage change: +0.15 🎉

Comparison is base (ebf5c41) 82.34% compared to head (484388a) 82.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
+ Coverage   82.34%   82.50%   +0.15%     
==========================================
  Files         124      127       +3     
  Lines        3098     3149      +51     
  Branches      732      744      +12     
==========================================
+ Hits         2551     2598      +47     
+ Misses        332      327       -5     
- Partials      215      224       +9     

Impacted Files	Coverage Δ
...lin/com/amazon/ionschema/reader/internal/ranges.kt	82.22% <77.27%> (-5.28%)	⬇️
...in/com/amazon/ionschema/model/ConsistentDecimal.kt	85.71% <85.71%> (ø)
.../com/amazon/ionschema/model/ConsistentTimestamp.kt	85.71% <85.71%> (ø)
...a/reader/internal/constraints/ValidValuesReader.kt	93.33% <93.33%> (ø)
...amazon/ionschema/reader/internal/TypeReaderV2_0.kt	92.64% <100.00%> (+0.10%)	⬆️

... and 3 files with indirect coverage changes

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[desaikd]

très bien 👍🏻