adds changes for implementation of ISL ranges

Issue #10

Description of changes: This PR works on creating ISL ranges.

Changes:

adds changes for ISL Range
- RangeBoundaryValue: represents a range boundary value (i.e. min, max or a value in terms of RangeBoundaryValueType)
- RangeBoundaryValueType: represents a range boundary value which can be a Decimal/Integer/...
- RangBoundaryType: defines exclusivity of the range boundary
adds a bugfix for unordered types comparison (a new PartialEq implementation for IslTypeImpl)
adds unit tests related to ranges

Test: Adds unit tests for ISL Ranges

amazon-ion / ion-schema-rust