Integer type policy

Integers are for numbers, enabling arithmetic like subtractions and for
loops without getting shot in the foot. Unsigneds are for bitfields.

- "int" for numbers that will always be laughably smaller than four
  billion, and where we don't care about the serialization format.

- "int32" for numbers that will always be laughably smaller than four
  billion, and will be serialized to four bytes.

- "int64" for numbers that may approach four billion or will be
  serialized to eight bytes.

- "uint32" and "uint64" for bitfields, depending on required number of
  bits and serialization format. Likewise "uint8" and "uint16", although
  rare in this project since they don't exist in XDR.

- "int8", "int16" and plain "uint" are almost never useful.

internal/model/puller_test.go

@@ -199,7 +199,7 @@ func TestCopierFinder(t *testing.T) {
 	// Update index
 	m.updateLocal("default", existingFile)
 
-	iterFn := func(folder, file string, index uint32) bool {
+	iterFn := func(folder, file string, index int32) bool {
 		return true
 	}
 
@@ -264,7 +264,7 @@ func TestCopierFinder(t *testing.T) {
 
 // Test that updating a file removes it's old blocks from the blockmap
 func TestCopierCleanup(t *testing.T) {
-	iterFn := func(folder, file string, index uint32) bool {
+	iterFn := func(folder, file string, index int32) bool {
 		return true
 	}
 
@@ -341,7 +341,7 @@ func TestLastResortPulling(t *testing.T) {
 	// with a different name (causing to copy that particular block)
 	file.Name = "newfile"
 
-	iterFn := func(folder, file string, index uint32) bool {
+	iterFn := func(folder, file string, index int32) bool {
 		return true
 	}