Collection fixes

[Pentiva]

This stops the source generator from crashing with a NullReferenceException for ICollection types.
GetEnumeratorSymbol now falls back to implemented interfaces when the provided type symbol does not define one. This method was returning null when given an IReadOnlyCollection and causing the whole generation to fail.
TryGetGenericCollectionInterface was also expanded to support all related collection interfaces.

Tests for both ICollection and IReadOnlyCollection were also added, but are probably overkill, as it was just the generation that was failing and might not be a necessity for ~300 extra tests.

[cathei]

Thank you for the contribution! I'll include it in next release.

[cathei]

I have had some changes with your implementation. I'll leave the note here.

• There was issue that ICollection is not in System.Collections.Generic but System.Collections. Fixed this.
• As GetEnumeratorSymbol update does not cover explicit interface implementation, updated this accordingly. It will require additional effort for code generation to fall back explicit interface implementation. It is todo for now.
• I separated test case to dedicated ones to test generation.