adds implementation of `valid_values` constraint

[desaikd]

Issue #9 #10

Description of changes: This PR works on adding implementation of valid_values constraint.

Grammar:

<VALID_VALUES> ::= valid_values: [ <VALUE>... ]
                 | valid_values: <RANGE<NUMBER>>
                 | valid_values: <RANGE<TIMESTAMP>>

Ion Schema specification: https://amzn.github.io/ion-schema/docs/spec.html#valid_values

List of changes:

• adds changes for Number range implementation (https://github.com/amzn/ion-schema-rust/pull/92/commits/d184e29462a756524e68c98d1a46767a7a1ace63) (Note: Timestamp ranges are not completely implemented yet, refer #91)
• adds ValidValues enum variants for IslConstraint and Constraint
• adds ValidValuesConstraint implementations
• adds validation logic for valid_values

Tests: added unit tests for valid_values implementation.

• adds tests for programmatically creating valid_values constraint
• adds tests for loading a schema using valid_values constraint
• adds validation tests for valid_values constraint

Question: Should there be a Violation returned when an OwnedElement with annotation is passed to validate with valid_values constraint or silently ignore the annotation? (Note: current implementation ignores the annotation)

[popematt]

Should there be a Violation returned when an OwnedElement with annotation is passed to validate with valid_values constraint or silently ignore the annotation? (Note: current implementation ignores the annotation)

No. The current implementation is correct. This constraint is intended to compare the value, excluding annotations. In fact, it is even illegal to use annotations on the valid values arguments, with the exception of ranges (e.g. valid_values: [foo::1] is illegal).

Good question, though. 👍🏼