Improve Oid documentation and make Oid::from const fn

[jannschu]

See rusticata/asn1-rs#20.

[chifflier]

Note: this breaks rustfmt (I can fix it).

To avoid that, I suggest to add something to your .git/hooks/pre-push file.I use:

#!/bin/sh 
remote="$1"                                                                        
url="$2"

if [ -f Cargo.toml ] && cargo sync-readme -V >/dev/null 2>&1; then
    echo "[+] check sync-readme"
    if ! cargo sync-readme -c; then
        exit 1
    fi
fi
if [ -f Cargo.toml ] && cargo fmt --version >/dev/null 2>&1; then
    echo "[+] check rustfmt"
    if ! cargo fmt --all -- --check; then
        exit 2
    fi
fi

exit 0

[chifflier]

Merged, with some modifications (rustfmt), in f0bd012 and 3d309c5 Thanks!