Add arrays of struct instances (StructType[N] vars)

Parser: accept `Type[N] name` between type and identifier
(Prog8ANTLR.g4, Antlr2KotlinVisitor) via elementToArray()
for STRUCT_INSTANCE; CodeDesugarer: handle struct-instance
bases, addressOfStructInstance(), whole-struct memcopy
(points[a]=points[b]), and fix variable-index 16-bit scaling
in 6502 AssignmentAsmGen.computeStructArrayElementAddress();
IR: preserve STRUCTDEFS round-trip, VM loader support;
m68k/new6502/intermediate backends; docs + todo updated.
Limitations: not as struct fields (AstChecker/
AstIdentifiersChecker error) nor as sub params.

codeGenIntermediate/src/prog8/codegen/intermediate/ExpressionGen.kt

@@ -582,7 +582,7 @@ internal class ExpressionGen(private val codeGen: IRCodeGen) {
 
     private fun translate(arrayIx: PtArrayIndexer): ExpressionCodeResult {
         if(arrayIx.type.isStructInstance)
-            throw AssemblyError("cannot translate POINTER[x] resulting in a struct instance; this is likely part of a larger expression POINTER[x].field and that has to be translated earlier as a whole")
+            throw AssemblyError("cannot translate array[x] resulting in a struct instance; this is likely part of a larger expression array[x].field and that has to be translated earlier as a whole")
 
         val eltSize = codeGen.program.memsizer.memorySize(arrayIx.type, null)
         val vmDt = codeGen.irType(arrayIx.type)