Shouldn't split/duplicate SInvokables

[smarr]

Currently, BlockNodes split SInvokable objects. This seems wrong and highly problematic for two reasons:

• SInvokable are language-level objects. Splitting/duplicating causes issues with object identity, and exposes implementation-level concerns.
• Blocks are checked for identity based on their methods, thus, this will cause problems in dispatch chains and unnecessarily pollute caches.

Relevant code bits:

• BlockNode
• BlockDispatchNode (just one of multiple)

This is also an issue in TruffleSOM, likely in Grace @richard-roberts and TruffleMate @charig.