Constant-type pointers, readonly ports

[kiedtl]

The idea is that pointers are marked as const using some method, and are then prohibited from being passed to words that (indirectly or directly) call STA (i.e. takes a non-const pointer).

("Marking" pointers in this case means modifying their type signature. I'm not settled on what the signature for this is -- could be @const Type, @@Type, &Type, (Cons @Type), or something else.)

Pointers can be made const in a few ways:

• (const) to constify the pointer on TOS
• (const foo) to constify a variable
• #const (let CONSTANT [ 0x0 ]) to declare a variable constant.
  • Not a fan of this one, as it encourages abusing variables to hold constant values (words should be used for that).

Same principle for device ports -- writing a readonly port literal results in Const_Dev16/Const_Dev8, which cannot be passed to words that require a non-readonly port. (Not sure if adding this particular feature is worth the extra complexity. I mean, how often would one accidentally try to write to a readonly port?)

[kiedtl]

Should probably use different terminology to differentiate from const words, which will be added at some point (to enable compile-time calculations etc).

immut/readonly would be good for this.

• (immutify)
• (immutify foo)