mapping.scm and hashx-remove!

[Kevin Ryde]

In ice-9/mapping.scm line 97, hash-table-mapping-hooks has a call

        (hashx-get-handle hash-proc assoc-proc delete-proc table key)

which is the wrong number of arguments for that function.

Just from looking at it, I wonder if it's meant to be hashx-remove!
instead of hashx-get-handle.

Except there's no hashx-remove! defined.  scm_hashx_remove_x exists,
but it's not given a scheme level binding.

Looks like scm_hashx_remove_x might be missing "&closure" for
scm_hash_fn_remove_x too, though again that's only from looking at it.