Prefix bin+sep to add-bin dest argument

[sogaiu]

This PR is an attempt to address #1484.

With this change, one should be able to use bundle/add-bin like this:

(bundle/add-bin manifest
               "git-some-janets.janet"
               # dest argument
               "git-some-janets")

instead of having to express the same intent like this:

(bundle/add-bin manifest
                "git-some-janets.janet"
                # dest argument
                "bin/git-some-janets")

That is, if the optional dest argument is specified to bundle/add-bin, it will have bin and an appropriate separator prefixed to it before being passed to install/add-file.

I tested the new version with spork and one of my own repositories and it seems to work.

[pepe]

LGTM