[src/core/IO.pm] nicer &dir

No change 'inside', only 'outside'. Works the same way.
'/' is unnecessary in particular calls.

before:
> .Str.say for dir './'
.//file2
.//file1
.//dir2
.//dir1

after:
> .Str.say for dir './'
./file2
./file1
./dir2
./dir1