This branch seems pretty stable, and it lets us get rid of some
legacy code about extensions.

We used to have lol::map::empty() to empty maps, but in std::map
this method is called clear(). It sounds safe to harmonise between
our types and have lol::array::clear() too.

bullet3, pegtl: update upstream branches.
imgui: add missing files in “make dist”.

The only reason we had to keep this was for emscripten builds. We can
now use SDL v2 using “emconfigure ./configure CFLAGS="-s USE_SDL=2” and
a few additional flags.

SDL_Mixer v2 is not available in emscripten packages but either it will
be added at some time, or I will get rid of it altogether. An alternate
solution could be this code snippet I found:

    cd SDL2_mixer
    emconfigure ./configure --prefix=$(pwd)/dist --enable-music-ogg --disable-music-ogg-shared CFLAGS="-s USE_VORBIS=1"
    emmake make install

Previous code relied on preprocessor check for INT_MAX != PTRDIFF_MAX but
it is a lot more reliable to use std::enable_if instead.

This new version allows to compute odd and even double factorials
without harming performance for the standard factorial.

Most operators do not yet handle reals of different sizes, but
this change fixes the most glaring bugs.

It was hard to avoid some of the compiler optimisations caused by our
intermediate conversion to double, so we hide them behind the actual
real object. Also, this commit fixes a potential exponent overflow.

Now that the exponent is no longer biased, it is a signed number, and we
need to account for that when dividing it and expecting rounding rules to
behave in a certain way.

Some checks are failing, I probably messed up several functions.

This is the beginning of the removal of lol::String which is not really
useful once all its tiny utility functions are implemented for std::string.