Prevent ELIF or ELSE after an ELSE

Fixes #749

test/asm/elif-after-else.asm

@@ -0,0 +1,18 @@
+if 0
+	println "zero"
+else
+	println "one"
+	if 1
+		println "A"
+	else
+		println "B"
+	elif 2
+		println "C"
+	else
+		println "D"
+	endc
+elif 2
+	println "two"
+else
+	println "three"
+endc

test/asm/elif-after-else.err

@@ -0,0 +1,2 @@
+FATAL: elif-after-else.asm(14):
+    Found ELIF after an ELSE block

test/asm/elif-after-else.out

@@ -0,0 +1,2 @@
+one
+A

test/asm/multiple-else.err

@@ -0,0 +1,2 @@
+FATAL: multiple-else.asm(11):
+    Found ELSE after an ELSE block

test/asm/skip-elif-condition.asm

@@ -0,0 +1,17 @@
+mac: MACRO
+	if (\1) < 10
+		println "small \1"
+	elif (\1) > 100
+		println "large \1"
+	elif (\1) / 0 == 42 ; only evaluated if the "large" condition was taken
+		println "division by zero!?"
+	elif syntax! error?
+		println "X_X"
+	else
+		println "unreachable"
+	endc
+ENDM
+
+	mac 2 + 2
+	mac STRLEN("abcdef")
+	mac 101

test/asm/skip-elif-condition.err

@@ -0,0 +1,2 @@
+FATAL: skip-elif-condition.asm(17) -> skip-elif-condition.asm::mac(6):
+    Division by zero

test/asm/skip-elif-condition.out

@@ -0,0 +1,3 @@
+small 2 + 2
+small STRLEN("abcdef")
+large 101