Compile error in `examples/unit.rs`

[nnethercote]

cargo test --all-targets produces this compile error:

error[E0407]: method `is_valid` is not a member of trait `super::Conversion`
  --> examples/unit.rs:13:1
   |
13 | / unit! {
14 | |     system: uom::si;
15 | |     quantity: uom::si::length;
...  |
18 | | }
   | |_^ not a member of trait `super::Conversion`
   |
   = note: this error originates in the macro `unit` (in Nightly builds, run with -Z macro-backtrace for more info)

[yamilbknsu]

I'm experiencing the same error when trying to implement a unit into the SI system. I found that in src/units.rs is_valid is implemented as if it were part of the Conversion<V> trait, yet it is not defined in lib.rs

Update: this should be fixed in #521