preserve func expression to make Func.String() faster.

maybe better to revisit it's worth doing.

before:
% ./run_integration_test.rb android
tar -xzf ../android.tgz
Running make for android... 5.78 secs
Running kati for android... 62.50 secs
android: OK

PASS!

after:
% ./run_integration_test.rb android
tar -xzf ../android.tgz
Running make for android... 5.85 secs
Running kati for android... 61.93 secs
android: OK

PASS!
3 files changed