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/db/set_test.go

@@ -43,7 +43,7 @@ func genBlocks(n int) []protocol.BlockInfo {
 		for j := range h {
 			h[j] = byte(i + j)
 		}
-		b[i].Size = uint32(i)
+		b[i].Size = int32(i)
 		b[i].Hash = h
 	}
 	return b