FIXED: issue looking for symbols in local env
FIXED: issue copying parameters at function entrance
ADDED: instant value loading
ADDED: mulop support
FIXED: issue arranging variables for compound statements
ADDED: relop
FIXED: bug when adjusting stack pointer in assignment
FIXED: local_env switch in compound statement
ADDED: if
ADDED: supporting global variable
ADDED: array
FIXED: bug dealing with array variable
ADDED: iteration(unfinished)