Allow Spin2 structs to access their parent's symbols

Test/Expect/stest314.p2asm

@@ -0,0 +1,51 @@
+con
+	_clkfreq = 20000000
+	_clkmode = 16779595
+	ARR_SIZ = 12
+dat
+	nop
+	cogid	pa
+	coginit	pa,##$404
+	orgh	$10
+	long	0	'reserved
+	long	0 ' clock frequency: will default to 20000000
+	long	0 ' clock mode: will default to $100094b
+	orgh	$400
+ _ret_	mov	result1, #0
+	org	0
+entry
+
+_zb
+	add	arg02, arg01
+	rdbyte	result1, arg02
+	add	result1, #12
+_zb_ret
+	ret
+builtin_bytefill_
+        shr	arg03, #1 wc
+ if_c   wrbyte	arg02, arg01
+ if_c   add	arg01, #1
+        movbyts	arg02, #0
+builtin_wordfill_
+        shr	arg03, #1 wc
+ if_c   wrword	arg02, arg01
+ if_c   add	arg01, #2
+        setword	arg02, arg02, #1
+builtin_longfill_
+        wrfast	#0,arg01
+        cmp	arg03, #0 wz
+ if_nz  rep	#1, arg03
+ if_nz  wflong	arg02
+        ret
+
+result1
+	long	0
+COG_BSS_START
+	fit	480
+	orgh
+	org	COG_BSS_START
+arg01
+	res	1
+arg02
+	res	1
+	fit	480

Test/stest314.spin2

@@ -0,0 +1,8 @@
+{$Spin2_v46}
+
+CON
+  ARR_SIZ = 12
+  STRUCT st( byte a[ARR_SIZ] )
+
+PUB zb(^st b, i) : r
+  r := b.a[i] + sizeof(b)

frontends/spin/spin.y

@@ -250,8 +250,11 @@ SpinDeclareStruct(AST *ident, AST *defs)
         current->subclasses = P;
         P->superclass = current;
         P->fullname = current->fullname; // for finding "class using"
-        newobj->d.ptr = (void *)P;
+         newobj->d.ptr = (void *)P;
         P->isPacked = 1;
+
+        /* import parent's symbols into P */
+        P->objsyms.next = &current->objsyms;
     }
 
     PushCurrentModule();