Added an optional scope argument to expr_uses_var(s). This allows the user to pass in a scope of exprs bound to variables, e.g. from let expressions. The visitor will dive into the bound expressions, whenever a variable in scope is encountered.
Added comments to the new versions of expr_uses_var(s). Added EXPORT macros to expr_uses_var for Windows support.
Merge pull request #545 from halide/scoped_expr_uses_vars

Added an optional scope argument to expr_uses_var(s).