A small function formatting a file size into the nearest ISO representation fitting into 4 characters.

Examples: ``` use filesize::fit4;

asserteq!(&fit4(999), "999"); asserteq!(&fit4(12345), "12K"); asserteq!(&fit4(999999), "1.0M"); asserteq!(&fit4(7155456789_012), "7.2T"); ```