lint(vfs): don't hide elided lifetime

Ref:
```
Compiling vfs v0.0.0 (/build/source/vfs)
error: hiding a lifetime that's elided elsewhere is confusing
--> vfs/src/lib.rs:73:17
|
73 | pub fn iter(&self) -> impl Iterator<Item = VfsEntry> {
| ^^^^^ -------- the same lifetime is hidden here
| |
| the lifetime is elided here
|
= help: the same lifetime is referred to in inconsistent ways, making the signature confusing
= note: `-D mismatched-lifetime-syntaxes` implied by `-D warnings`
= help: to override `-D warnings` add `#[allow(mismatched_lifetime_syntaxes)]`
help: use `'_` for type paths
|
73 | pub fn iter(&self) -> impl Iterator<Item = VfsEntry<'_>> {
| ++++

error: could not compile `vfs` (lib) due to 1 previous error
```