Replace std::function with hyper_function for a 37% speedup in compile time

[Quuxplusone]

commit f6f96e1

Fix some minor std::function-related bugs.

Lambdas to be stored in `function<void()>` should not return `bool`.

Two uses of `std::function` could be just `function`, like everywhere
else in the codebase.

commit 4b3b2fd

Replace std::function with hyper_function for a 37% speedup in compile time.

Before:

    time c++ -O2 -DMAC -I/usr/local/include -std=c++11 -march=native
             -W -Wall -Wextra -Werror -pedantic -Wno-format-pedantic
             -Wno-missing-field-initializers -Wno-unused-parameter
             -DCAP_GLEW=0 -DCAP_PNG=0    -c hyper.cpp -o hyper.o

    real 2m22.508s
    user 2m20.625s
    sys  0m1.648s

After:

    time c++ -O2 -DMAC -I/usr/local/include -std=c++11 -march=native
             -W -Wall -Wextra -Werror -pedantic -Wno-format-pedantic
             -Wno-missing-field-initializers -Wno-unused-parameter
             -DCAP_GLEW=0 -DCAP_PNG=0    -c hyper.cpp -o hyper.o

    real 1m30.515s
    user 1m29.793s
    sys  0m0.689s

Comparing object file size:

    -rw-r--r--  1 ajo  staff  8215036 Jan  5 20:46 old-hyper.o
    -rw-r--r--  1 ajo  staff  7547232 Jan  5 20:47 new-hyper.o

Comparing number of symbols:

    nm old-hyper.o | wc -l  => 12590
    nm new-hyper.o | wc -l  =>  9742

No appreciable difference in link time; the linker takes less than
half a second in either case.

[Quuxplusone]

Interestingly, I did a global sed to replace = [] with = +[] and the compile time went back up from 1m30s to 1m50s (even as the number of distinct symbols in the nm output dropped from 9742 to 9132). I don't know how to explain that increase in compile time.

I've verified that this change would be acceptable to MSVC according to the AppVeyor build; although the AppVeyor build is currently broken for some reason related to SDL include paths.

[zenorogue]

Wow, that is quite a big improvement. Although on my computer it is much less substantial (~2%). It does not compile on some older compilers I am using, so I have added USE_STDFUNCTION for now. Thanks!