particle/map: new particle; maps a tag value to a particle

This particle is basically a dictionary mapping tag values to
particles.

particles/map.h

@@ -0,0 +1,11 @@
+#pragma once
+#include "../particle.h"
+
+struct particle_map {
+    const char *tag_value;
+    struct particle *particle;
+};
+
+struct particle *particle_map_new(
+    const char *tag, const struct particle_map *particle_map, size_t count,
+    struct particle *default_particle, int left_margin, int right_margin);