Implement String#unpack

[seven1m]

I split this issue off from #217.

We implement these in StringUnpacker.

• unpack1_spec.rb
• a_spec.rb
• at_spec.rb
• b_spec.rb
• comment_spec.rb
• c_spec.rb
• d_spec.rb
• e_spec.rb
• f_spec.rb
• g_spec.rb
• 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
• u_spec.rb
• v_spec.rb
• w_spec.rb
• x_spec.rb
• z_spec.rb

[seven1m]

Closing!! 🎉