Reinstate RL into the _RS family

Removal of colon-less labels lifted the grammar ambiguity that
prevented `RL` from being usable as a variable declarator.
Thus, reinstate its functionality.

test/asm/rs.asm

@@ -0,0 +1,43 @@
+
+a1 rb 1
+a2 rw 1
+a3 rl 1
+
+PRINTLN "a1 = ", a1
+PRINTLN "a2 = ", a2
+PRINTLN "a3 = ", a3
+PRINTLN "_RS = ", _RS
+
+
+b1 rb 1
+b2 rw 1
+b3 rl 1
+
+PRINTLN "b1 = ", b1
+PRINTLN "b2 = ", b2
+PRINTLN "b3 = ", b3
+PRINTLN "_RS = ", _RS
+
+
+rsset 42
+c1 rb 1
+c2 rw 1
+c3 rl 1
+
+PRINTLN "c1 = ", c1
+PRINTLN "c2 = ", c2
+PRINTLN "c3 = ", c3
+PRINTLN "_RS = ", _RS
+
+
+rsreset
+d1 rb 1
+d2 rw 1
+d3 rl 1
+
+PRINTLN "d1 = ", d1
+PRINTLN "d2 = ", d2
+PRINTLN "d3 = ", d3
+PRINTLN "_RS = ", _RS
+
+

test/asm/rs.out

@@ -0,0 +1,16 @@
+a1 = $0
+a2 = $1
+a3 = $3
+_RS = $7
+b1 = $7
+b2 = $8
+b3 = $A
+_RS = $E
+c1 = $2A
+c2 = $2B
+c3 = $2D
+_RS = $31
+d1 = $0
+d2 = $1
+d3 = $3
+_RS = $7