and remove redundant check in grisu2_prettify.

Change refs to fmt/time.h -> fmt/chrono.h
RST label rename time-api -> chrono-api

This reduces symbol sizes and gets rid of shadowing warnings.

Error:
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packagecloud.io/github/git-lfs/ubuntu trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6B05F25D762E3157
W: http://ppa.launchpad.net/couchdb/stable/ubuntu/dists/trusty/Release.gpg: Signature by key 15866BAFD9BCC4F3C1E0DFC7D69548E1C17EAB57 uses weak digest algorithm (SHA1)
W: Failed to fetch https://packagecloud.io/github/git-lfs/ubuntu/dists/trusty/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6B05F25D762E3157
W: Some index files failed to download. They have been ignored, or old ones used instead.