utils.py: Wrap calls to os.path.realpath() in an LRU cache

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.