Revert "Part-way done with adding AST to recursive descent"

This reverts commit 12efc96a901deff94ebd99740ba3b45fd6dc7fe1.
Revert "Revert "Part-way done with adding AST to recursive descent""

This reverts commit baee15771c4f9211284e6b2e6663ab4775f5cdbc.
Finished AST and program. created AST Comparison Tester

As far as I know, all of the pa1 tests compile and build a AST tree. I
wrote a comparison script to check with other people's Compilers, but I
haven't tested it with anyone yet.