3392: Implement concat eager macro r=matklad a=edwin0cheng
This PR implements the following things:
1. Add basic eager macro infrastructure by introducing `EagerCallId` such that the new `MacroCallId` is defined as :
```
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum MacroCallId {
LazyMacro(LazyMacroId),
EagerMacro(EagerMacroId),
}
```
2. Add `concat!` builtin macro.
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
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