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/scanner/blocks.go

@@ -46,7 +46,7 @@ func Blocks(r io.Reader, blocksize int, sizehint int64) ([]protocol.BlockInfo, e
 		}
 
 		b := protocol.BlockInfo{
-			Size:   uint32(n),
+			Size:   int32(n),
 			Offset: offset,
 			Hash:   hf.Sum(nil),
 		}