pre-RFC `const` function arguments

[gnzlbg]

@crlf0710

This pre-RFC will just kill thus one-to-one mapping. So function arguments will no longer have a direct mapping to the unstable std::ops::Fn::call method, which currently takes such an tuple as its parameter…

Per this pre-RFC const function arguments desugar into generic functions with const-generic parameters, that is, the tuple being passed to std::ops::Fn::call does not contain a const in it.