Add is_* and as_* methods to the event enums

[joshka]

Often application code only cares about a small subset of possible
events. These methods make it simpler to write code which checks whether
an event is a particular event type or converts events into the specific
type (returning an Option).

A simple example of this is waiting for any key, which now becomes:

while !event::read()?.is_key_press() {}

This can help simplify some nested match blocks. E.g.:

match event {
    Event::Key(key) if key.kind == KeyEventKind::Press => { ... }
	_ => {}
}

becomes:

if let Some(key) = event.as_key_press() { ... }

Similar flexible methods are aded across all the event enums:

• Event::is_focus_gained()

• Event::is_focus_lost()

• Event::is_key()

• Event::is_mouse()

• Event::is_paste()

• Event::is_resize()

• Event::is_key_press()

• Event::as_key_press() -> Option<&KeyEvent>

• MouseEventKind::is_*()

• MouseButton::is_*()

• KeyEventKind::is_*()

• KeyEvent::is_press()

• KeyEvent::is_release()

• KeyEvent::is_repeat()

• KeyCode::is_*()

• KeyCode::is_function_key(n)

• KeyCode::is_char(c)

• KeyCode::as_char() -> Option<char>

• KeyCode::is_media_key(media)

• KeyCode::is_modifier(modifier)

[joshka]

There is an in-flight PR in derive_more that would be also be useful. It adds an AsVariant derive. This would make it easy to deal with e.g. if let Some(mouse) = event.as_mouse() { ... } etc. I'd like to consider waiting for that and adding it to this PR.

Obviously a proper match statement is much better when dealing with more than one event type, but there are many use cases that this change would make simpler.

[joshka]

The derive_more change seems stalled for now, so I added the as_* functions manually.