Removed m*_stack_* opcodes, make them default behaviour

Memory operations used operands to encode positional/size arguments,
with stack variants in case the user wanted to programmatically do so.
In most large scale cases I don't see the non-stack variants being
used; many cases require using the stack for additions and
subtractions to create values such as indexes or sizes.  Therefore,
it's better to be stack-first.

One counter point is inline optimisation of code at runtime: if an
compile-time-known object is pushed then immediately used in an
operation, we can instead encode the value directly into an operand
based instruction which will speed up execution time because it's
slower to pop the value off the stack than have it available as part
of the instruction.

lib/inst.h

@@ -64,31 +64,16 @@ typedef enum
   OP_MALLOC_HWORD,
   OP_MALLOC_WORD,
 
-  OP_MALLOC_STACK_BYTE,
-  OP_MALLOC_STACK_SHORT,
-  OP_MALLOC_STACK_HWORD,
-  OP_MALLOC_STACK_WORD,
-
   OP_MSET_BYTE,
   OP_MSET_SHORT,
   OP_MSET_HWORD,
   OP_MSET_WORD,
 
-  OP_MSET_STACK_BYTE,
-  OP_MSET_STACK_SHORT,
-  OP_MSET_STACK_HWORD,
-  OP_MSET_STACK_WORD,
-
   OP_MGET_BYTE,
   OP_MGET_SHORT,
   OP_MGET_HWORD,
   OP_MGET_WORD,
 
-  OP_MGET_STACK_BYTE,
-  OP_MGET_STACK_SHORT,
-  OP_MGET_STACK_HWORD,
-  OP_MGET_STACK_WORD,
-
   OP_MDELETE,
   OP_MSIZE,