Check constancy when calling to external function #1480
What I did
How I did it
The compiler was not checking constancy of external calls inside of a constant context. However, it was correctly generating static calls, so calls to modifying functions would fail at runtime when the callee tried to modify state (per https://github.com/ethereum/EIPs/blob/d95612c8206b7e225bec5dc34c4b019c9531e44d/EIPS/eip-214.md). This adds a check when compiling external calls that they cannot be modifying within a constant context.
How to verify it
Description for the changelog
Add constancy check for external calls
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