demo crate not valid identifier

[softprops]

hi @dtolnay I ran into another issue after moving from mashup to lando. I'll open a separate issue but rather than pointing you through more complex code Ill wanted to whittle this down to a smaller case in a though away pull for testing

in short I'm trying to prefix my own macros with $crate:: allow for 2018 style macro imports but inside a paste::item! invocation I found that I'm not able to do so because I'll get the following error

    = help: message: `"$crate"` is not a valid identifier

[softprops]

this currently fails with this error message

error: proc macro panicked
   --> tests/test.rs:154:9
    |
154 | /         paste::item! {
155 | |             $crate::crate_not_valid_identifier! { [<Lib env!("CARGO_PKG_NAME")>] }
156 | |         }
    | |_________^
...
162 |      crate_not_valid_identifier!();
    |      ------------------------------ in this macro invocation
    |
    = help: message: `"$crate"` is not a valid identifier

[softprops]

$crate:: is needed to enable 2018 style imports

[dtolnay]

Thanks, closing in favor of #3 to keep the discussion in one place.