clean-up some u64 sizes and replace with u32

[garyyu]

clean-up some u64 sizes and replace with u32 or u8 for those vector length and so on.

1. CompactBlockBody 3 vector length:

   
   pub struct CompactBlockBody {
   /// List of full outputs - specifically the coinbase output(s)
   pub out_full: Vec<Output>,
   /// List of full kernels - specifically the coinbase kernel(s)
   pub kern_full: Vec<TxKernel>,
   /// List of transaction kernels, excluding those in the full list
   /// (short_ids)
   pub kern_ids: Vec<ShortId>,
   }

2. `MerkleProof` path vector length:
```Rust
pub struct MerkleProof {
    /// The size of the MMR at the time the proof was created.
    pub mmr_size: u64,
    /// The sibling path from the leaf up to the final sibling hashing to the
    /// root.
    pub path: Vec<Hash>,
}

3. TransactionBody 3 vector length:

   pub struct TransactionBody {
   /// List of inputs by the transaction.
   pub inputs: Vec<InputEx>,
   /// List of outputs the transaction produces.
   pub outputs: Vec<Output>,
   /// List of kernels that make up this transaction (usually a single kernel).
   pub kernels: Vec<TxKernel>,
   }

4. MsgHeader msg_len. This one we still use u64 in the structure definition but use u32 in the serialization/deserialization. At this moment, I'm not sure whether it's a good idea to directly replace this msg_len with u32 type, I will check it later.

pub struct MsgHeader {
    magic: [u8; 2],
    /// Type of the message.
    pub msg_type: Type,
    /// Total length of the message in bytes.
    pub msg_len: u64,
}