first commit
added files until chapter 5, but need testing
Merge branch 'master' of https://github.com/sunchao/tiger
updated semant.sml in chap5.
added chap5/driver.sml for testing
added checking for duplicated definitions
added checking for cyclic mutually recursive definition
add more checkings, now can check all testcases
added files for chap 6
added findescape.sml
testing remote
moved slp.sml
Merge branch 'master' of git://github.com/sunchao/tiger