Add new fluent builders and iterators for `InputMap`

[Shute052]

• added new fluent builders for creating a new InputMap<A> with short configurations:
  • fn with(mut self, action: A, input: impl Into<Item = UserInput>).
  • fn with_one_to_many(mut self, action: A, inputs: impl IntoIterator<Item = UserInput>).
  • fn with_multiple(mut self, bindings: impl IntoIterator<Item = (A, UserInput)>) -> Self.
  • fn with_gamepad(mut self, gamepad: Gamepad) -> Self.
• added new iterators over InputMap<A>:
  • bindings(&self) -> impl Iterator<Item = (&A, &UserInput)> for iterating over all registered action-input bindings.
  • actions(&self) -> impl Iterator<Item = &A> for iterating over all registered actions.
• removed InputMap::build method in favor of new fluent builder pattern.
• renamed InputMap::which_pressed method to process_actions to better reflect its current functionality for clarity.