5.1.5: Revisit the support for floating point literals

[pietroalbini]

In chapter 5.1.5. Range Patterns

Issue exported from our legacy tracking system.

[jonas-schievink]

So I this seems to be about rust-lang/rust#41620. Realistically, the lint is (hopefully) never going to be set to deny-by-default or even become a hard error, because it makes a lot of perfectly fine code impossible to write (instead of just annoying, since you have to turn off that lint).

I think a reasonable rule here would be to accept floating-point literals in patterns (including negated literals), but not paths to constants (since those can be NaN or Inf), and specify a ..= b to match x iff a <= x && x <= b, and to never be exhaustive.

Realistically, we can also just treat this like any other future-incompat lint, since I don't think this is something the initial Ferrocene users are likely to want to write.