Bring some bugfixes from upstream (dropbox/json11).

Bring some bugfixes from upstream (dropbox/json11).

- change Json map/vector conversions to invoke begin() directly instead of
  using key_type/value_type/mapped_type, to prevent false positives with
  std::optional, which has a value_type member type, but is not a container
  (ec4e45219af1d7cde3d58b49ed762376fccf1ace)

- make has_shape() return true for NUL type only if element actually exists
  (e2e3a11e99672b018e0e0657867e6a3439e180cf)

- improve handling of errors in comments
  (3bafee93e6d587d5bc6ef362c3e4457688e0ed5b)

- make operator== and operator< faster by checking for node identity
  (dabb88e83ebd6d5ef765864ec6d77bb4451c115e,
   42e6e71abb460e06c83a8f17c02e9bdf7959d2e7)

Signed-off-by: Miod Vallat <miod.vallat@powerdns.com>