Implement mid-section alignment directive

Fixes #254.

test/asm/align-pc.asm

@@ -0,0 +1,22 @@
+SECTION "fixed org", SRAM[$BEAD]
+	ds 42
+	static_assert @ == $BED7
+	align 5,$17
+
+
+; Should land at $0001
+SECTION "align", ROM0,ALIGN[1,1]
+	db 69
+	align 1 ; This wants to go at $0000, $0002, $0004...
+
+
+; Should land at $0003
+SECTION "under-aligned", ROM0,ALIGN[1,1]
+	dw $BEEF
+	align 2,1 ; This wants to go at $0001, $0005, $0009...
+
+
+; Should land at $0005
+SECTION "forced align", ROM0
+	dw $DEAD
+	align 2,3 ; This wants to go at $0003, $0007, $000B...