Use section numbers when resolving local labels

This changes the semantics of local labels. Previously, local labels
were just shorthand for writing out `name.local`, which meant if there
were multiple sections named "name", there could potentially be name
collisions.

This commit adds a unique numeric suffix to each section's ID string.
This guarantees that locals in one section won't interfere with those in
a different section.

src/labels/snapshots/zasm__labels__process__test__local_labels.snap

@@ -18,3 +18,38 @@ expression: board_to_text(board)
 '
 :stop
 #walk i
+---
+'Using locals before any globals are defined
+:_loop1
+#take gems 1 _break
+#_loop1
+:_break
+---
+'Multiple sections with the same name
+#end
+:touch
+Trying to sell you some ammo...
+#take gems 10 _skip
+#give ammo 10
+:_skip
+#zap touch
+#end
+:touch
+Trying to sell you some torches...
+#take gems 15 _skip0
+#give torches 5
+:_skip0
+#restore touch
+---
+'Multiple locals with the same name
+#end
+:repeat3
+:_z
+:_z
+:_z
+#take gems 1 _skip1
+#give score 1
+:_skip1
+#zap _z
+#_z
+:_z