symbol.c: use chunk-based pool for symbol string allocation

Replace per-symbol mrb_malloc() with a chunk-based string pool that
batches allocations into 4KB chunks. This reduces malloc call count
by ~12x (e.g. 909 vs 10,887 for 10k dynamic symbols) and eliminates
per-allocation malloc metadata overhead (~16 bytes/symbol).

Pool allocations are rounded up to even size to preserve LSB pointer
tagging used for literal detection.

Co-authored-by: Claude <noreply@anthropic.com>

include/mruby.h

@@ -301,6 +301,7 @@ typedef struct mrb_state {
   const char **symtbl;
   size_t symcapa;
   struct mrb_sym_hash_table *symhash;
+  void *sym_pool;
 #ifndef MRB_USE_ALL_SYMBOLS
   char symbuf[8];                         /* buffer for small symbol names */
 #endif