New lint: `filter_map_identity`

[camsteffen]

What it does

It's the same as flat_map_identity, but with filter_map.

Example

vec![Some(1)].into_iter().filter_map(|x| x)

Could be written as:

vec![Some(1)].into_iter().flatten()

[magurotuna]

Hi! I'm interested in this so I'd like to give it a shot.

[magurotuna]

@rustbot claim