Precompilation significantly slows Julia startup

Julia startup time with PyCall precompiled into base/userimg.jl jumps in commit d19995c5fd895aa6a249cb6b6d1164dc2826e8f2 from <1 second to ~10 seconds on several of my machines (with both Julia 0.3.6 and some 0.4 masters).

I found this with a git bisect using a quick and dirty script:

``` bash
#!/bin/bash
# GOOD PyCall: 896f82a
# BAD PyCall: ebe352f

JIFILE=/tmp/test_bisect
julia /home/jason/src/julia/contrib/build_sysimg.jl $JIFILE native /tmp/userimg.jl --force > /dev/null

T1=`date +%s`
/bin/time -f %U julia -J ${JIFILE}.ji -E "0"
TIME=$(echo `date +%s`-$T1 | bc -l)
echo $TIME

if [[ `echo "$TIME>2" | bc -l` -eq 1 ]]; then
echo "Long"
exit 1
else
echo "Short"
exit 0
fi
```

I'm not actually sure if this is a problem with PyCall or Julia, so if the latter, then let me know and I'll move the bug up the stack.

I don't know enough about PyCall to debug this further, but I did do some macro level debugging in [this mailing list thread](https://groups.google.com/forum/#!topic/julia-dev/uvEgky_QKmY). At the time I did not know that it was from PyCall specifically.

Oh, and another gotcha: the slow start seems to be dependent on stdout. Let me show you what I mean:

``` julia
$ /bin/time -f %U julia -J /tmp/test_newgit.ji -E 0 # slow start
0
9.03
$ julia -J /tmp/test_newgit.ji -E 0 >/dev/null # fast start
0.63
$ julia -J /tmp/test_newgit.ji -e 0 # fast start
0.59
$ julia -J /tmp/test_newgit.ji -e "println(3)" # slow start
3
9.40
```

Whereas with a commit before d19995c5fd895aa6a249cb6b6d1164dc2826e8f2 (precompiled), all of these would be fast.

Start with the reported regression commits d19995c5fd895aa6a249cb6b6d1164dc2826e8f2, 896f82a, and ebe352f, then inspect contrib/build_sysimg.jl and reproduce the timing with the supplied bash script. Compare startup with stdout redirected and with different expressions, and use the linked mailing-list investigation to isolate whether PyCall or Julia causes the slowdown. Done means identifying the regression and restoring fast precompiled startup.

索引モデルが issue の本文から書いたものです。