* acts as a safe-guard against uninterrupted infinite loops
   or other operations that are costly to undo in interactive mode.
   If we're out of memory, it is usually too late to react properly.
   This implementation tries to avoid OOMs due to SciTECO behaviour.
   We cannot fully exclude the chance of an OOM error.
 * The undo stack size is only approximated using the
   UndoToken::get_size() method.
   Other ways to measure the exact amount of allocated heap
   (including size fields in every heap object or using sbrk(0) and
   similar) are either costly in terms of memory or platform-specific.
   This implementation does not need any additional memory per heap
   object or undo token but exploits the fact that undo tokens
   are virtual already. The size of an undo token is determined
   at compile time.
 * Default memory limit of 500mb should be OK for most people.
 * The current limit can be queried with "2EJ" and set with <x>,2EJ.
   This also works interactively (a bit tricky!)
 * Limiting can be disabled. In this case, undo token processing
   is a bit faster.
 * closes #3

 * session.hg sets up the buffer session in the current
   Mercurial repository
 * session.vcs is a convenience macro that may be used in
   profiles to enable buffer sessions per repo for all supported
   VCS (Git, Hg and SVN)

 * it uses "svn info --xml" since otherwise the output of "svn info" might
   be localized.

this allows you to use sciteco in a Git repository to edit a specific file,
without changing the buffer session. Also useful if SciTECO is used as the GIT_EDITOR
without thrashing the repository's session every time a commit message is edited.

 * $SCITECOCONFIG has been introduced, so have a macro-accessible location
   for the profile, buffer session etc.
   This is set to the program dir on Windows. That way, the config files
   will be found, regardless of the current working dir, but it may also
   be set up for Unix-like environments on Windows.
 * $SCITECOPATH defaults to the program dir + "/lib" now on Windows.
 * The default profile is now always called ".teco_ini". Also on Windows.
   Platform differences like this would need to be documented.
 * The sample teco.ini has been renamed to "sample.teco_ini" for clarity