xFunc 4.4.0-preview.2

Changes:

• 857c840 #708 - Remove unnecessary variables.
• ba25ecd #706 - CallExpression uses incorrect scope for parameters evaluation.
• 0fffdba #704 - Lambda can't resolve a parameter.
• 66d962b #701 - Align units ToString implementation with parser.
• ba0a7fb #667 - Migrate to NUnit.
• bdcb5e1 #696 - Remove Helpers from public API.
• 44396df #175 - Support rational numbers
• a80d477 DocFx Documentation
• 3b696ef Remove ThrowHelper
• 0bb61e4 #680 - Remove Moq.