Use correct 64 bit type in jit write barrier

This was causing an issue on 32 bit systems where it would use the wrong
type.
Add spec for rest argument for Method#parameters
Fix handling of unnamed splat

This also uses the :* name, so if anyone ever uses unnamed_splat as
name, we don't collide with that. :* should not be possible to use since
that would be the unnamed splat itself.
Add specs for proc parameters with block arguments

Also adds specs for method parameters and blocks for the same case.
Fix proc parameters handling with a block argument

Fixes #2396
Add tag for failing spec