Tree walker fails all bad test cases (closes #37, #43, #46)
Testing some main() checking semantic checks (#36)
Added functionality to get a TigerFunction's id
Main checks work, now need to embed in parse grammar (#36)
Main function checks successfully embedded in parse grammar (closes #36)
'void main()' must now be the last signature declared in a tiger_program
Merge pull request #47 from CS4240-tiger/main-func-check

Main function checks complete