This is important since Sys_var_typelib and its descendants return
pointers to constant symbols from *_value_ptr, which are situated in
write-protected-memory.

* functions const-qualified:
  - value_ptr
  - session_value_ptr
  - global_value_ptr
  - default_value_ptr
  - Sys_var_vers_asof::value_ptr
  - other minor private ones

* remove C-style typecasts when it discards qualifiers