Implement Array#pack

[seven1m]

1. Remove "skip-test" comment from top of a spec/core/array/pack/*_spec.rb file.
2. Modify ArrayPacker and/or StringPacker to make the specs pass
3. Commit! (EASY! :laughing:)

• [x] a_spec.rb
• [x] at_spec.rb
• [x] b_spec.rb
• [ ] buffer_spec.rb
• [x] comment_spec.rb
• [x] c_spec.rb
• [ ] d_spec.rb
• [x] empty_spec.rb
• [ ] e_spec.rb
• [ ] f_spec.rb
• [ ] g_spec.rb
• [x] h_spec.rb
• [ ] i_spec.rb
• [ ] j_spec.rb
• [ ] l_spec.rb
• [ ] m_spec.rb
• [ ] n_spec.rb
• [ ] percent_spec.rb
• [ ] p_spec.rb
• [ ] q_spec.rb
• [ ] s_spec.rb
• [x] u_spec.rb
• [ ] v_spec.rb
• [ ] w_spec.rb
• [ ] x_spec.rb
• [x] z_spec.rb