`has_length` on `Map` does not need `PartialEq` constraint on the value

[woshilapin]

has_length is constrained by V: PartialEq + Debug but the fact that the value has PartialEq constraint is useless to count entries in the map. It is actually a problem in the following example.

struct Object(u32);

let mut map = HashMap::new();
map.insert(0, Object(42));

assert_that!(map).has_length(1);

This code produces the following error

error[E0599]: no method named `has_length` found for type `spectral::Spec<'_, std::collections::HashMap<{integer}, tests::from_gtfs_into_model::Object>>` in t
he current scope
   --> gtfs/src/lib.rs:132:27
    |
132 |         assert_that!(map).has_length(1);
    |                           ^^^^^^^^^^ method not found in `spectral::Spec<'_, std::collections::HashMap<{integer}, tests::from_gtfs_into_model::Object>>`
    |
    = note: the method `has_length` exists but the following trait bounds were not satisfied:
            `spectral::Spec<'_, std::collections::HashMap<{integer}, tests::from_gtfs_into_model::Object>> : spectral::hashmap::HashMapAssertions<_, _>`

error: aborting due to previous error

A simple fix is the implementation of PartialEq on Object but that's not really on option for some objects.