Updates in line with recent Snap-Core changes

Changes Snap.Internal.Types to Snap.Internal.Core

Related to: https://github.com/snapframework/snap-core/commit/02c087b91b1aafdc865b9f19d12a9ed770667bcb
Merge pull request #64 from ChristopherBiscardi/snap-core-internal

Updates in line with recent Snap-Core changes