Deprecate SET in favor of =

`SET` is redundant with `=`, and is already the name of an instruction.

test/asm/def.asm

@@ -1,10 +1,10 @@
 def variable = 1
 	println variable
-def variable set 2
+def variable = 2
 	println variable
 redef variable = 3
 	println variable
-redef variable set 4
+redef variable = 4
 	println variable
 
 DEF constant EQU 42

test/asm/symbol-override.asm

@@ -1,14 +1,14 @@
-V set 0
-V set 1
+V = 0
+V = 1
     PRINTLN "V={V}"
 
 W equ 1
-W set 0
+W = 0
 
 rsset 1
 X rb 0
-X set 0
+X = 0
 
 SECTION "Test", ROM0[1]
 Y:
-Y set 0
+Y = 0