os: add disk_usage()

[kbkpbot]

disk_usage(path) returns disk usage of path
Examples:

pub struct DiskUsage {
pub:
        total     u64
        available u64
        used      u64
}

usage := os.disk_usage('.')!
assert usage.total > 0
assert usage.available > 0
assert usage.used > 0

[huly-for-github]

Connected to Huly®: V_0.6-22058

[spytheman]

Excellent work.