[ruby/yarp] Remove varargs entirely

Following up on ruby#1483, this removes our usage of varargs entirely.
Under the new approach we make explicit methods for the various
signatures. Because we know the type of everything any it's very
consistent, this makes it easier for compilers to inline.

Also, I renamed the functions to be more consistent with each other
so that we'll have some more internal consistency.

ruby/prism@87af9ba29b