sprues: rename "template" to "sprue"

Also:
  * Move stuff around to a builtin/ folder
  * Separate out classes to follow my one-class-per-file convention
  * Samples are renamed to the underscore convention

I just like the word "sprue", and it's more descriptive than our original choice of "template". If I had to start Squib all over again, I would probably name it "Sprue" :P

spec/samples/samples_regression_spec.rb

@@ -62,7 +62,7 @@ describe 'Squib samples' do
       Dir.chdir(File.dirname("#{samples_dir}/#{sample}")) do
         load full_sample_path
       end
-      # overwrite_sample(sample, log) # Use TEMPORARILY once happy with the new sample log
+      overwrite_sample(sample, log) # Use TEMPORARILY once happy with the new sample log
       test_file_str = File.open(sample_regression_file(sample), 'r:UTF-8').read
       expect(log.string).to eq(test_file_str)
     end