New definition syntax with leading DEF keyword

This will enable fixing #457 later once the old
definition syntax is removed.

test/asm/def.asm

@@ -0,0 +1,29 @@
+def variable = 1
+	println variable
+def variable set 2
+	println variable
+redef variable = 3
+	println variable
+redef variable set 4
+	println variable
+
+DEF constant EQU 42
+	println constant
+
+DEF string EQUS "here"
+	println "{string}"
+
+rsreset
+def _x rb
+def _y rw 2
+def _z rl
+def _size rb 0
+	println "{_x} {_y} {_z} {_size}"
+
+def constant equ 6*7 ; fails
+	println constant
+
+redef string equs "there"
+	println "{string}"
+
+redef constant equ 6*9 ; syntax error