`generate_const` on generator and `ImplMod`

[VictorKoenders]

it would be nice to be able to generate compile-time checks. These are usually in the form of:

const _: () = { if !some_check() { panic!(""); }};

this does not work in impl Foo { .. } blocks as rust will not allow underscore names on associated consts.

We should have a copy of generate_const on both the generator (top level) and ImplMod (nested mod)

[stale[bot]]

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