Refactor code, add ASTNode and ASTTerm classes

ast/arrays.py

ast/assign.py

ast/ast_node.py

ast/bin_op.py

ast/block.py

ast/branches.py

ast/cast.py

ast/lit.py

ast/loops.py

ast/math.py

ast/memory.py

ast/operators.py

ast/properties.py

ast/select.py

ast/sizeof.py

ast/transform.py

ast/utils.py

ast/variables.py

code_gen/cgen.py

graph/graphviz.py