fix incorrect inlining of + when + is locally redefined

fix incorrect inlining of + when + is locally redefined

* libguile/vm-i-scheme.c (FUNC2): Use a signed value for the intermediate
  result here. Not sure what the effect is, though.

* module/ice-9/psyntax.scm (chi-top): Toplevel definitions ensure that
  variables are defined in the current module. Fixes the specific case of
  guile-lib's md5.scm, which redefines + -- this code is needed so that
  we don't incorrectly open-code +.

* module/language/tree-il/primitives.scm (resolve-primitives!): I think
  there were some cases in which vars and names would not resolve
  properly here. Fix those.