Speed up decoding of oversized integers

Bigint.from_msgpack_ext rebuilt the integer by shifting and adding one
32-bit chunk at a time. Each step copies the whole growing bignum, so
decoding was quadratic in the payload length (a 1 MiB ext payload took
over 20 seconds). Convert the chunks to a hex string and parse it in a
single String#to_i(16), which runs in linear time.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Watson1978

lib/msgpack/bigint.rb

@@ -5,7 +5,8 @@ module Bigint
     # We split the bigint in 32bits chunks so that individual part fits into
     # a MRI immediate Integer.
     CHUNK_BITLENGTH = 32
-    FORMAT = 'CL>*'
+    CHUNK_FORMAT = 'L>*'
+    FORMAT = "C#{CHUNK_FORMAT}"
 
     if Integer.instance_method(:[]).arity != 1 # Ruby 2.7 and newer
       # Starting from Ruby 2.7 we can address arbitrary bitranges inside an Integer with Integer#[]
@@ -56,12 +57,7 @@ def self.from_msgpack_ext(data)
       parts = data.unpack(FORMAT)
 
       sign = parts.shift
-      sum = parts.pop.to_i
-
-      parts.reverse_each do |part|
-        sum = sum << CHUNK_BITLENGTH
-        sum += part
-      end
+      sum = parts.reverse.pack(CHUNK_FORMAT).unpack1('H*').to_i(16)
 
       sign == 0 ? sum : -sum
     end