fix(util/lean_path.cpp): get_exe_location follows symbolic link in OSX

Previously, we had different behaviors on Linux and OSX when
get_exe_location finds a location of executable:

 - Linux: follow symbolic link
 - OSX: not follow symbolic link
refactor(library/data/list): cleanup, rename concat to assoc
fix(frontends/lean/parser): segmentation fault after REPLACE, fixes #172