Fix wrong assertion

Closes #174 
I don't think you can do simplier than I did.

Your code gives me:
```rust
lib.rs:346:49: 346:66 error: too many type parameters provided: expected at most 0 parameter(s) but found 1 parameter(s) [E0087]
lib.rs:346         debug_assert!(nv <= std::num::Bounded::<mesh::VertexCount>::max_value() as uint);

lib.rs:346:29: 346:88 error: the type of this value must be known in this context
lib.rs:346         debug_assert!(nv <= std::num::Bounded::<mesh::VertexCount>::max_value() as uint);
```