Adding support for --version flag

which also involved extracting the common version info parts used later
on for `RUBY_` constants and such.
Using the Ruby space as a memo

instead of passing around/memoizing values from yet another function.  I
*think* this is more in keeping with other parts of topaz, and I hope is
more like what @alex would want :-)
Directly fetching const

rather than spinning up an execution context.  Not happy about the
duplication.  Meh.
Folding the const assignments back into `_entry_point`

since it felt kinda weird and wasn't saving much, but was adding a ton
to the generated C.  I'm still not sure if I should even think about
looking at the generated C...
Using local var directly

now that it's available again.
Changes per feedback.

I totally should have looked at `test_verbose`...  derp.
Merge pull request #384 from modcloth/add-version-flag

Add version flag