Sam Thursfield authored Jan 19, 2018 and Jürg Billeter committed Feb 12, 2018

The os.path.realpath() function is expensive and we call it many times,
to the point that os.path.realpath() calls make up around 40% of the
total time spent in Element.stage_artifact().

The cleanest way to fix this is with a `functools.lru_cache()` wrapper
that caches recently used values. None of the code in question can be
removed (as the tests added in the previous commit will demonstrate).

I tested this by running `bst shell base/base-system.bst true` in
the GNOME modulesets project.

  o Without this patch there are 240,019 calls os.path.realpath()

  o With this patch there are 10,379 calls to os.path.realpath()

  o If we increase the cache size to 128 items, there are 10,359 calls
    to os.path.realpath().

  o If we reduce the cache size to 32 items, there are 10,426 calls.

  o In all cases the number of *unique* calls is 10,327.

This fixes issue #174.