LexCore introduced for embedding core in lexical language
this will ultimately go away when locals moves back into desugar, but it is necessary right now because it is plain too slow to actually desugar exception handling.  interpreting the program "3" with some lexCore hand-coding takes 22 seconds.  expressing the same logic in lexical took 3 minutes.
re-coding locals logic in core
this makes things fast enough to actually test again.
Merge branch 'master' of github.com:brownplt/lambda-py
merge for new LexApp desugaring