Added homework 4, fix for what I got wrong in hw3
Finished going through Eloe's pdf
Figured out how the %type thing works
Working on type-checking for BIN_OP...
Oops...
Working on N_BIN_OP error handling
Finished N_BIN_OP
Handling meta-types correctly now
Finished LAMBDA, T_NIL, T_T
Finished N_IF_EXPR
IT COMPILES

Symbol table lookup works now
Finished tying up loose ends- N_LET_EXPR has a type now
All the tests pass!