Support (SETF name) as function names

[davazp]

Currently we have a hack to support (defun (setf name) ...), but (setf X) doesn't work as proper functions. They can't be used with fdefinition or function.

Suggestions / Ideas

We should support a register of SETF functions. This can be implemented as a hash table or maybe just adding a setfFunc field to symbols.

This probably requires a few adjustments to the compiler anyway, so it could be worthy to have the support right in the compiler so it's available from the beginning?

[vlad-km]

The easiest way to implement such a setf-name registry is to put a special system tag with a bit mask on the name (unification of system types - see https://github.com/jscl-project/jscl/issues/391#issuecomment-1036555699)

Using a hash-table is trickier. It is impossible to raise the standard hash-table to the boot level. It is possible to implement a system hash-table on pure js (prelude.js) and api at the compiler level. It is not yet clear here whether such objects should be tagged. Probably yes.

I would suggest the first option for starters.