Reuse `j` for `key`.

Save 3 bytes by eliminating a variable in the interpreter. I don't feel
so bad about reusing `j` to mean `key` because `j` is an index in the
other loops and it's an index in the loop where it replaces `key`.

See #46.

File                             Raw         Min         Zip     Min/Zip
index.js                  5348/05.22  2022/01.97  1814/01.77   984/00.96