Drop rustc-serialize dependency (fix #8)

[tatsuya6502]

• Remove direct/indirect dependencies to rustc-serialize crate because it has been deprecated: https://users.rust-lang.org/t/deprecation-of-rustc-serialize/10485

  • For RMP (Rust MessagePack): replace rmp-serialize (rustc-serialize binding) with rmp-serde (Serde binding). This crate is used to serialize/deserialize key and metadata stored in RocksDB.
  • Replace rust-crypto crate with md-5 and data-encoding generic-array crates. These crates are used to calculate MD5 checksum for blob.
  • Replace direct dependency to rustc-serialize with data-encoding. This crate is used to convert raw digest bytes to hexadecimal String.

• Change HunkFlag::NoMd5 to HunkFlag::NoChecksum. We will soon replace MD5 checksum with a safe hasher as of today such as SHA-256, SHA-3-256, or Blake2s).

• Change the return type of put() from io::Result<Etag> to io::Result<()>.

[tatsuya6502]

The CI passed. Merging.