Conversions

[baseTwo]

Fix for issue #466 Continued from PR #469

• Adds an out parameter to ChangeType to indicate the result of the conversion. This prevents us from needing to have a CanConvert or similar function, which would result in duplicative work. In all cases where ChangeType would result in NoMatch, an exception is thrown.
• In the As operator, if there is a custom conversion defined between two types, we invoke it. If ChangeType identifies a NoMatch condition, we return default. Otherwise, we produce a standard C# as operator.
• ChangeType throws an error when binding to Convert fails. Instead, use TryConvert to the binder.

[EvanMachusak]

Tested by:

• CqlToElm/AsTest/FhirId_As_FhirString
• CqlToElm/AsTest/Choice_As