Reduce maintenance burden: needs refactoring

[fredbi]

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Refactoring proposal:

• all assert package code moves to internal/assertions, including tests
• package assert and require become 100% codegenenerated

Deprecated methods are produced by codegen. No need to maintain those in core internal package.
Tests generated in assert and require

Next step (separate issue) : codegen generics versions

[fredbi]

completed with #8