Fix errors in expression type checking.

  * ast_expr.cc: Add compatibility check to equality expression and
    assignment expression. Fix few issues with array, field and method/
    call expressions. Arguments checking of Call is not implemented.

Signed-off-by: Hongjiu Zhang <zhanghj@umich.edu>
Fix compiling errors

Signed-off-by: Hongjiu Zhang <zhanghj@umich.edu>