Use a runtime check for Macro.classify_atom/1

[zachallaun]

We ran into an issue in Lexical reported here: lexical-lsp/lexical#805

Lexical is packaged by precompiling everything, usually on the lowest supported version of Elixir/OTP, and then injecting those compiled modules when launching the runtime node where user project code runs. This means that compile-time version checks could lead to an unexpected code branch at runtime, as the runtime might be a later version.

I think this would usually be okay, since compile-time version checks are usually there to use new APIs and fall back to ones that would still work, but in this case, a private API (Code.Identifier.classify) was being used that no longer exists in 1.14+.

I know this code-path is performance-sensitive, but a quick Benchee shows that function_exported?/3 is a small fraction of the cost of Version.match?/2:

Name                              ips        average  deviation         median         99th %
using_function_exported       12.31 M      0.0812 μs ±10125.72%      0.0800 μs       0.100 μs
using_version_check            0.26 M        3.84 μs   ±605.70%        3.28 μs        7.38 μs

Comparison:
using_function_exported       12.31 M
using_version_check            0.26 M - 47.28x slower +3.76 μs

I've confirmed locally that this change addresses the issue linked above.

[zachallaun]

If interested, here's the benchmark I used:

Mix.install([
  :benchee
])

defmodule Test do
  def using_function_exported do
    function_exported?(Macro, :classify_atom, 1)
  end

  def using_version_check do
    Version.match?(System.version(), ">= 1.14.0-dev")
  end
end

Benchee.run(
  %{
    "using_function_exported" => &Test.using_function_exported/0,
    "using_version_check" => &Test.using_version_check/0
  },
  time: 10,
  memory_time: 2
)