Fix typo.
Delete comment.
`wffi-define-all`: handle lib name simply. Closes #2.

Automatically treat the `lib-name` as relative to the path of the
source file using the `wffi-define-all` form. For example in
`my-api.rkt` you can simply write:

    (wffi-define-all "my-api.md" <before> <after>)

and if `my-api.md` is in the same directory as `my-api.rkt` it will
just work.

The contortions @takikawa had to use (for both phases 0 and 1) are no
longer required. (To avoid breaking such code, the
`current-markdown-files-path` parameter is still `provide`d, but it's
a deprecated no-op.)
LIB-ID should be based on LIB-NAME not lib-path.

In other words

    (wffi-define-all "foo.md" ....)

should expand to

   (define foo-lib (wffi-lib #<path:/path/to/foo.md>))

as it did before my previous commit accidentally changed it, _not_ to

   (define /path/to/foo-lib (wffi-lib #<path:/path/to/foo.md>))

Although when using `wffi-define-all` people aren't likely to need to
refer to the lib itself, if they do need to, it should be the same
short, predictable name as before.