Refactor #generate_api_method to improve performance

[jkowens]

This is an attempt to resolve #1672.

Benchmark script:

require 'grape'
require 'benchmark/ips'

Benchmark.ips do |ips|
  ips.report('class_load') do
    class API < Grape::API
      prefix :api
      version 'v1', using: :path
      get '/' do
        'hello'
      end
    end
  end
end

Before:

Warming up --------------------------------------
          class_load   590.000  i/100ms
Calculating -------------------------------------
          class_load      5.589k (±10.3%) i/s -     27.730k in   5.029428s

After:

Warming up --------------------------------------
          class_load   860.000  i/100ms
Calculating -------------------------------------
          class_load      8.761k (±13.6%) i/s -     43.000k in   5.035511s

[grape-bot]

	1 Warning
⚠️	There’re library changes, but not tests. That’s OK as long as you’re refactoring existing code.

Generated by 🚫 danger

[dblock]

I think this code is an MRI 1.8 artifact, since 0e0a94f was fixing some issue there and we haven't touched it for 5 years, so LGTM.

[dblock]

I think this deserves a CHANGELOG entry. Maybe something like "Improved startup performance during API method generation"?

[jkowens]

I've added that changelog entry, thanks!

[dblock]

Merged, thank you.