Made a number of small changes to enable building on CoreFx

- Remove uses of MemoryStream::GetBuffer.  This API isn't supported on
CoreFx. The TryGetBuffer is available but in these cases seemed better
to just call Stream::CopyTo
- Changed Type.Assembly to Type.GetTypeInfo().Assembly
- Moved scripting APIs out of CommonCompiler and into
ScriptCompilerUtil.  These methods relied on APIs that are not available
on CoreFx and have no equivalent.
- Minor API updates.