This fixes #24.

[bugfix] The var_subst_map is a member of codegen_state not of the ecm

When using CInstructions with plain CTarget() as code generation target,
an AttributeError was thrown when evaluating the if clause here.
This commit uses the var_subst_map of the codegen_state instead.

Among other things, this ensures that impl_arg_info is picklable.