Should we make `nested_tuple.0.0` legal syntax?

[matklad]

@CAD97

How does name.0.name lex and parse today? Naively, I'd expect the 0. to be lexed as a float currently. (On mobile, can't check.)

Special cased in the lexer: https://github.com/rust-lang/rust/blob/ea52be482ab4945fda63cb65b6a198309a041e3c/src/librustc_lexer/src/lib.rs#L453-L457