[TASK] Explore `Str` interning

[kurtlawrence]

Goal

R&D interning of Strs for equality performance improvement.

The pattern of referencing column headers lends itself to an interner for equality. This might improve performance when working with strings, but should be validated.

• Interned strings could be stored sorted, this makes string comparison faster
• Interned map only needs to last for evaluation phase
• Consider whether Str is altered, or leverage the properties of Arc for equality