More work on improving stacktraces slightly.

Add extra information about when we change fibers. The janet
stack is really a spaghetti stack, where each fiber represents
a group of stack frames as well as a place where we can longjmp to. It
is therefor useful information for the programmer to know where each
stack frame is.

However, an argument could be made that this clutters the stackframe
and is more of a hindrance than a help.

src/core/compile.c

@@ -1056,7 +1056,7 @@ JanetCompileResult janet_compile_lint(Janet source,
 
     if (c.result.status == JANET_COMPILE_OK) {
         JanetFuncDef *def = janetc_pop_funcdef(&c);
-        def->name = janet_cstring("_thunk");
+        def->name = janet_cstring("thunk");
         janet_def_addflags(def);
         c.result.funcdef = def;
     } else {