Most IInternable implementations can already be represented as ReadOnlySpan<char> and are thus redundant. The StringBuilder based implementations are problematic from performance point of view in that they're indexing into the StringBuilder as iterating over its characters could be an O(N^2) operation.

We should:

• Delete IInternable.
• Convert the non-StringBuilder uses to ReadOnlySpan<char>.
• Figure out how to address StringBuilder implementations.
  • On .NET Core we could use the new GetChunks() method.
  • Or we could simply convert the StringBuilder to string and verify that the perf win outweighs the cost of the string allocation.