message: add way to create a block builder from a block
diff: @@ -20,6 +20,15 @@ pub struct BlockBuilder {
  pub contents: BlockContents,
  }
  +impl From<Block> for BlockBuilder {
  + fn from(block: Block) -> BlockBuilder {
  + BlockBuilder {
  + common: block.header.common,
  + contents: block.contents,
  + }
  + }
  +}
  +
  /// block builder to build and finalize the construction of a block
  impl BlockBuilder {
  /// default setting, equivalent to writing a genesis block (the empty block)
  