Add assertions

Closes #292

test/asm/assert.asm

@@ -0,0 +1,23 @@
+SECTION "fixed", ROM0[0]
+
+FixedBase:
+	assert FixedBase ; This should eval (and fail) at compile time
+
+	ds 0
+	static_assert @ == 0, "@ ain't 0 now? (Hint: it's {@})"
+
+	ds 42
+	assert WARN, @ - FixedBase != 42 ; This should also eval at compile time
+
+SECTION "floating", ROM0
+
+FloatingBase:
+	assert FAIL, FloatingBase == 0 ; This shouldn't eval at compile time
+
+	ds 4
+	static_assert FAIL, FloatingBase != 0 ; This is not constant!
+
+	ds 69
+	static_assert FATAL, FixedBase != 0 ; This will fail... ↓
+	; The point of `FATAL` is for stuff that can, say, cause division by 0!
+	static_assert FAIL, 1 / FixedBase, "You dun goofed, son" ; Won't be read

test/asm/assert.err

@@ -0,0 +1,10 @@
+ERROR: assert.asm(4):
+    Assertion failed
+warning: assert.asm(10): [-Wassert]
+    Assertion failed
+ERROR: assert.asm(18):
+    Expected constant expression: 'FloatingBase' is not constant at assembly time
+ERROR: assert.asm(18):
+    Assertion failed
+ERROR: assert.asm(21):
+    Assertion failed