Parse::Matroska::Reader: get rid of `bigrat`

It seems that it was causing issues with certain perl setups (such as
the one on issue #549). It also turns out that it was not behaving correctly
(not all constants were being promoted to big nums as they should), so we
use explicit objects to derive the constants.

There were also precedence issues. I wonder if this even worked right to
begin with.

The 'double' path (8-byte floats) is untested, as I couldn't easily find
a file with such a field.

Closes #549.
Merge branch 'master' of git://github.com/mpv-player/mpv

* 'master' of git://github.com/mpv-player/mpv:
  win32: restore support for exe directory as config directory
  crosscompile-mingw: improve instructions for MXE
  sd_lavc: handle subtitles with no subtitle resolution set
  options: make --no-config block all auto-loaded configuration files
  lua: auto-load scripts from ~/.mpv/lua/
  lua: make register_event() not overwrite previous event handler