[Truffle] Get rid of the copy of options stored in RubyContext.

We used to copy some of the options into RubyContext where they can be
final for the benefit of the PE, as using Option.load() requires a
volatile read.
[Truffle] Remove visibility asserts - a mess, and don't always make sense in multi-language environment.
[Truffle] Change the generic dispatch node from caching to looking up every time.

This node had lots of potential problems - invalidation wasn't checked,
dead entries weren't removed, and the hash tables weren't synchronised.
For now we'll just remove the caching, call it UncachedDispatchNode,
and make it neverPartOfCompilation.
[Truffle] Remove accurate source positions for now.