Allow inserting a function on any type in a call chain:
```
any_type
.fun(|o| do_something_with(o))
.other_things();
```
More detailled example:
```
use {
have::Fun,
itertools::*,
};
fn analyze(log_lines: Vec<LogLine>) {
log_lines
.iter()
.into_group_map_by(|ll| ll.remote_addr)
.fun(|g| println!("{} distinct remote_addresses", g.len()))
.into_iter()
.sorted_by_key(|e| Reverse(e.1.len()))
.take(3)
.for_each(|e| println!("{} hits by remote addresse {}", e.1.len(), e.0));
}
```
## Usage:
```TOML
[dependencies]
have = "0.1"
```