Use charpool to back string_pool

string_pool (string interning) was using STL strings with lots of extra
constructions, when all we need is a const char array. We can use
charpool for that and get all the benefits there of tightly-packed heap
allocations.

charpool.h

@@ -66,7 +66,9 @@
 #define CHARPOOL_H
 
 void *cp_alloc(int sz);
-char *cp_strdup(const char *src);
+/* len does not include null terminator */
+const char *cp_strndup(const char *src, int len);
+const char *cp_strdup(const char *src);
 
 void cp_free(void);