Avoid accessing symbol internals in call_dsubr_1 and DEVAL

Avoid accessing symbol internals in call_dsubr_1 and DEVAL

The symbol's characters are only accessed in case they are needed
for an error message.  This can be avoided by passing the symbol
all the way to a error message function.

* libguile/__scm.h (SCM_WTA_DISPATCH_1_SUBR): new macro

* libguile/error.c (scm_i_wrong_type_arg_symbol): new error function

* libguile/error.h: declaration of scm_i_wrong_type_arg_symbol

* libguile/eval.c (call_dsubr_1): use new macro SCM_WTA_DISPATCH_1_SUBR
  to avoid having to unpack the symbol's chars

* libguile/eval.i.c: use new macro SCM_WTA_DISPATCH_1_SUBR