small compiler performance improvement

Inference calls eval() a lot to resolve module references, and this was
inefficient since eval() assumes expressions might need to be expanded,
so everything was going back through part of the front end. Fixed by
calling the interpreter directly.

Unfortunately this does not affect the system image build time much,
only a couple of seconds. But these are worth it:

time for `make testall`

before:
real	3m56.272s
user	13m41.657s
sys	0m4.517s

after:
real	3m6.434s
user	10m52.977s
sys	0m4.223s

time for `using Gadfly`

before:
julia> tic(); using Gadfly; toc()
elapsed time: 51.968993674 seconds

after:
julia> tic(); using Gadfly; toc()
elapsed time: 48.591181748 seconds

Would be great to find more low-hanging fruit like this.