cli: Require explicit overflow-checks flag

[acheroncrypto]

Problem

overflow-checks flag is implicitly disabled by default.

Anchor workspaces that are crated with anchor init have this flag enabled by default but Anchor doesn't do any checks for it after workspace creation.

Summary of changes

• Require explicit overflow-checks flag in workspace Cargo.toml
• Add tests for add, sub, mul overflow cases
• Add overflow-checks = true in all CI tests
• Add a test to check whether anchor build fails when overflow-checks is not specified

Pros & Cons

Enabling overflow-checks not only allows to use regular math operations in a safe manner(and avoid checked_*().unwrap()) but it also works with dependencies i.e. a program will panic if a math operation in one of its dependencies overflows.

The only downside I can think of is that unchecked math operations cost a bit more compute units but this should be negligible.

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Closes #1759

[vercel]

@acheroncrypto is attempting to deploy a commit to the coral-xyz Team on Vercel.

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