feat: use const fn where possible

this just enables the use of the function inside other const contexts.
one potential caveat to consider is if you ever need to make this
method non-const in future, it's a breaking change.
that seems unlikely in this case, but it's worth bearing in mind.

suggested by clippy's `missing_const_for_fn` lint:
https://rust-lang.github.io/rust-clippy/master/index.html#/missing_const_for_fn

src/lib.rs

@@ -258,7 +258,7 @@ impl OfficeHours {
  /// }
  /// ```
  #[must_use]
- pub fn iter(&self) -> OfficeHoursIter {
+ pub const fn iter(&self) -> OfficeHoursIter {
  OfficeHoursIter {
  start: self.start,
  finish: self.finish,