Speed up immediate unpack when freeze: true by 10%

[skipkayhil]

Rename object_complete_symbol -> frozen

To use with other, non-symbol, pre-frozen objects.

---

Speed up immediate unpack when freeze: true by 10%

Since immediates are already frozen, we can skip over "maybe freeze"
behavior completely for those types.

ruby 4.0.2 (2026-03-17 revision d3da9fec82) +YJIT +PRISM [x86_64-linux]
Warming up --------------------------------------
             control     5.437k i/100ms
       frozen before     4.733k i/100ms
Calculating -------------------------------------
             control     54.830k (± 1.8%) i/s   (18.24 μs/i) -    277.287k in   5.057252s
       frozen before     49.078k (± 1.9%) i/s   (20.38 μs/i) -    246.116k in   5.014762s

Comparison:
 frozen after:    55009.0 i/s
      control:    54829.6 i/s - same-ish: difference falls within error
frozen before:    49078.3 i/s - 1.12x  slower

require 'msgpack'
require 'benchmark/ips'

data = []
100.times do
  data << rand(0..127)
  data << -rand(0..30)
  data << nil
  data << true
  data << false
  data << rand(0..65535)
end

packed = MessagePack.pack(data)

up_ctl = MessagePack::Unpacker.new(freeze: false)
up_frz = MessagePack::Unpacker.new(freeze: true)

Benchmark.ips do |x|
  x.report("control") { up_ctl.feed(packed); up_ctl.read }

  x.report("frozen #{ENV.fetch("STAGE", "after")}") { up_frz.feed(packed); up_frz.read }

  x.save!("/tmp/msgpack_un_freeze")

  x.compare!
end