Allow REDEF for EQU constants

[Rangi42]

Fixes #853

[ISSOtm]

The manual's modification is what I feared: it chains "REDEF can modify EQU" with "EQU can be used for unchanging properties"... Imo, the latter should be first, and then note that "if you really need to, you can use REDEF to modify one".

[Rangi42]

I've updated rgbasm(5) and given a more illustrative example for REDEF EQU.