I initially planned on doing that, but then was noticing that we access `RSpec::Mocks.space` _all over the place_ from the rest of the rspec-mocks code (e.g. to call `proxy_for` on it), and while `Array#last` shouldn't be a slow method call, my instinct is that it can only be faster to have `space` already available for direct access.  The cost of doing this is only the extra memory need for the `@space` ivar, but who cares about that?

Anyhow, putting it in the ivar also allows us to use `attr_reader`, which is apparently faster than any normal method definition could be:

https://github.com/rails/arel/commit/ea1d050266e76d0200f8ff749e537fc66c17ff4f