Release v2.0.0

Version 2.0.0

• Use once_cell instead of lazy_static for test
• Comply with Clippy rules
• More small fixes

The following change has been included since 2.0.0-beta.1.

• Use phf (perfect hash map) instead of ahash (#7, #9) by @mkroening
  Actual type of OEMCPHashMap will be changed.