Do we need `compact_string`?

[toddjonker]

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As noted in the binary encoding chapter:

• Do we need a compact_string encoding? It saves a byte for string lengths >16 and <128.

Personally, I'm inclined to add it.

• Strings in that length range are quite common. At scale, the byte-saving for shorter strings is meaningful.
• It improves symmetry with symbol, user's won't be surprised by its absence.
• Implementation is obvious and simple.