Add assertions

Closes #292

test/link/assert.asm

@@ -0,0 +1,11 @@
+
+SECTION "test", ROM0
+
+	ds 123
+
+FloatingBase:
+	assert WARN, FloatingBase & 0, "Worry about me, but not too much."
+	assert FAIL, FloatingBase & 0, "Okay, this is getting serious!"
+	assert FATAL, FloatingBase & 0, "It all ends now."
+	assert FAIL, FloatingBase & 0, "Not even time to roll credits!"
+	assert WARN, 0, "Still can finish the film, though!"

test/link/assert.out

@@ -0,0 +1,3 @@
+warning: assert.asm(7): Worry about me, but not too much.
+assert.asm(8): Okay, this is getting serious!
+error: assert.asm(9): It all ends now.