CIs update rust version to 1.78 today. Fix error

[cottinisimone]

Yesterday the CI run with 1.77. Today master branch run with 1.78 and a lint error occurred:

error: direct implementation of `ToString`
  --> positional/src/field.rs:49:1
   |
49 | / impl ToString for PositionalField {
50 | |     fn to_string(&self) -> String {
51 | |         self.value.pad(self.size, self.filler, self.alignment, true)
52 | |     }
53 | | }
   | |_^