rope: fix rope name template

Rope is a library to define a custom rope data
structure with specified type of a stored value, and
some rope functions such as split, alloc.

It is possible to choose a unique name for a defined
rope structure. It was implemented as

    #define rope_api(x) rope_##rope_name##_##x
    #define rope rope_##rope_name

But with such rope_api definition it was always
expanded to `rope_rope_name_<x value>`. So rope_name
was basically a constant 'rope_name' regardless what
was defined under it.

The patch fixes it and makes name generation just like
bps_tree.h.

Additionally, the name template is changed a bit, now
it is

    <rope_name>_ + rope + _<method>

instead of

    <rope> + _<rope_name>_ + <method>

It just appeared to look better. For example, consider
rope name 'xrow_update' and method 'size':

    new: xrow_update_rope_size()
    old: rope_xrow_update_size()

The second name would be generated by the old template
and looks wrong. The new name not only looks better,
but also conforms with our code style.