drw: minor improvement to the nomatches cache

1. use `unsigned int` to store the codepoints, this avoids waste on
   common case where `long` is 64bits. and POSIX guarantees `int` to be
   at least 32bits so there's no risk of truncation.
2. since switching to `unsigned int` cuts down the memory requirement by
   half, double the cache size from 64 to 128.
3. instead of a linear search, use a simple hash-table for O(1) lookups.

util.h

@@ -3,6 +3,7 @@
 #define MAX(A, B)               ((A) > (B) ? (A) : (B))
 #define MIN(A, B)               ((A) < (B) ? (A) : (B))
 #define BETWEEN(X, A, B)        ((A) <= (X) && (X) <= (B))
+#define LENGTH(X)               (sizeof (X) / sizeof (X)[0])
 
 void die(const char *fmt, ...);
 void *ecalloc(size_t nmemb, size_t size);