Bug-fix release 1.16.4.1

This release:

• adds validation to to and +to so they raise an exception if the user attempts to apply them to something other than a local variable or value, where previously if the user applied them to such a hardfault would likely occur at runtime.
• fixes a bug in extra/rp_common/picocalc_term_common.fs where to was applied to something other than a local variable or value in an uncommon code path.