Vyper `constant` removal creates 0.2.x to 0.3.0 compatibility boundary
The goal is to determine whether contracts written for Vyper 0.2.x that use the `constant` keyword can be recompiled under Vyper 0.3.0 or later without changing their on-chain storage layout or read behavior. In Vyper 0.2.x, `constant` declares immutable state variables backed by storage slots. Starting with Vyper 0.3.0, `constant` is no longer recognized; v
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