message: [types] Make `AccountAddress::new` a `const fn`
  This allows people to make a const `AccountAddress`. For example,
  ```rust
  const mock_acct: AccountAddress = AccountAddress::new([0u8; 16]);
  ```
  Closes:
  Approved by: phlip9
diff: @@ -31,7 +31,7 @@ const LIBRA_NETWORK_ID_SHORT: &str = "lb";
  pub struct AccountAddress([u8; ADDRESS_LENGTH]);
  impl AccountAddress {
  - pub fn new(address: [u8; ADDRESS_LENGTH]) -> Self {
  + pub const fn new(address: [u8; ADDRESS_LENGTH]) -> Self {
  AccountAddress(address)
  }
  