Rename all the `bucket's as `symbol_t'.

* src/gram.c, src/gram.h, src/lex.c, src/lex.h, src/output.c,
* src/reader.c, src/reader.h, src/reduce.c, src/state.h,
* src/symtab.c, src/symtab.h (bucket): Rename as...
(symbol_t): this.
(symbol_list_new, bucket_check_defined, bucket_make_alias)
(bucket_check_alias_consistence, bucket_pack, bucket_translation)
(bucket_new, bucket_free, hash_compare_bucket, hash_bucket)
(buckets_new, buckets_free, buckets_do): Rename as...
(symbol_list_new, symbol_check_defined, symbol_make_alias)
(symbol_check_alias_consistence, symbol_pack, symbol_translation)
(symbol_new, symbol_free, hash_compare_symbol_t, hash_symbol_t)
(symbols_new, symbols_free, symbols_do): these.