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

samples/sprues/my_sprues/fold_sheet.yml

@@ -0,0 +1,57 @@
+---
+sheet_width: 297mm
+sheet_height: 210mm
+card_width: 63.0mm
+card_height: 88.0mm
+cards:
+- x: 16.5mm
+  y: 13.0mm
+- x: 83.5mm
+  y: 13.0mm
+- x: 150.5mm
+  y: 13.0mm
+- x: 217.5mm
+  y: 13.0mm
+- x: 16.5mm
+  y: 109.0mm
+  rotate: :turnaround
+- x: 83.5mm
+  y: 109.0mm
+  rotate: :turnaround
+- x: 150.5mm
+  y: 109.0mm
+  rotate: :turnaround
+- x: 217.5mm
+  y: 109.0mm
+  rotate: :turnaround
+crop_line:
+  lines:
+  - type: :vertical
+    position: 16.5mm
+  - type: :vertical
+    position: 79.5mm
+  - type: :vertical
+    position: 83.5mm
+  - type: :vertical
+    position: 146.5mm
+  - type: :vertical
+    position: 150.5mm
+  - type: :vertical
+    position: 213.5mm
+  - type: :vertical
+    position: 217.5mm
+  - type: :vertical
+    position: 280.5mm
+  - type: :horizontal
+    position: 13.0mm
+  - type: :horizontal
+    position: 101.0mm
+  - type: :horizontal
+    position: 109.0mm
+  - type: :horizontal
+    position: 197.0mm
+  - type: :horizontal
+    position: 105.0mm
+    style: :dashed
+    color: :red
+    overlay_on_cards: true

samples/sprues/my_sprues/hex_tiles.yml

@@ -0,0 +1,24 @@
+---
+sheet_width: 210mm
+sheet_height: 297.000000mm
+card_width: 65.800000mm
+card_height: 76.000000mm
+cards:
+- x: 6.300000mm
+  y: 5.000000mm
+- x: 6.300000mm
+  y: 81.000000mm
+- x: 6.300000mm
+  y: 157.000000mm
+- x: 72.100000mm
+  y: 43.000000mm
+- x: 72.100000mm
+  y: 119.000000mm
+- x: 72.100000mm
+  y: 195.000000mm
+- x: 137.900000mm
+  y: 5.000000mm
+- x: 137.900000mm
+  y: 81.000000mm
+- x: 137.900000mm
+  y: 157.000000mm