perf: fix performance when applying large numbers of type unions

[HoneyryderChuck]

the handling of union types, particularly bad when used with records, creates transitive interfaces with the union of all methods from each option; a lot of these methods are common across records, but they wouldn't be deduped, thereby being union'ed several times while joined, to the point where a 12 member union could take +10m to load.

This fixes the behaviour by:

1: removing method overload dead pairs for [] calls for combinations
which cannot be called.
2: removing duplicates from the same overload, which weren't identified
due to pair ordering.

To reproduce this in the master branch, you can use the following script:

# Usage: N=8 ruby repro_union.rb [path/to/steep/checkout]
# Generates a union of N record types sharing a base record, then type-checks `frame[:stream]`.
require "tmpdir"
require "fileutils"

n = Integer(ENV.fetch("N", "6"))
steep = File.expand_path(ARGV[0] || "~/dev/steep")

Dir.mktmpdir("steep-union") do |dir|
  FileUtils.mkdir_p(%W[#{dir}/sig #{dir}/lib])
  members = (1..n).map { |i| "common & { type: :t#{i}, p#{i}: Integer, ?o#{i}: String }" }
  File.write("#{dir}/sig/a.rbs", <<~RBS)
    type common = { stream: Integer, flags: Integer }
    type frame = #{members.map { |m| "(#{m})" }.join(" | ")}
    class Foo
      def go: (frame) -> Integer
    end
  RBS
  File.write("#{dir}/lib/a.rb", "class Foo\n  def go(frame)\n    frame[:stream]\n  end\nend\n")
  File.write("#{dir}/Steepfile", %(target(:t) { signature "sig"; check "lib" }\n))

  t = Process.clock_gettime(Process::CLOCK_MONOTONIC)
  system({ "BUNDLE_GEMFILE" => "#{steep}/Gemfile" }, "bundle", "exec", "#{steep}/exe/steep", "check",
         "--jobs=1", "--steepfile=#{dir}/Steepfile", chdir: dir, out: File::NULL)
  puts "N=#{n}: #{(Process.clock_gettime(Process::CLOCK_MONOTONIC) - t).round(1)}s"
end