Allow mocking generic constructor methods

[asomers]

Currently Mockall can't mock a constructor method for a generic struct that has bounds on its generic parameters. For example:

mock! {
    pub Foo<T: Default +'static> {
        fn build<T2: Default + 'static>() -> MockFoo<T2>;
    }
}

produces errors like

error[E0277]: the trait bound `T2: std::default::Default` is not satisfied
  --> mockall/tests/mock_generic_constructor_2.rs:7:1
   |
7  | / mock! {
8  | |     pub Foo<T: Default +'static> {
9  | |         fn build<T2: Default + 'static>() -> MockFoo<T2>;
10 | |     }
11 | | }
   | |_^ the trait `std::default::Default` is not implemented for `T2`
   |
   = help: consider adding a `where T2: std::default::Default` bound