Allow numbers in literal/1

[greg-rychlewski]

There was a discussion where someone was using a database that does not allow query parameters in limit expressions but it's still desirable to have dynamic limit values. The simplest thing I can think of is to expand literal/1 to also allow numbers.

Some other considerations that I did not pursue:

• Should we allow literal outside of fragments? That would be a bigger change and would not really make sense with the original use case of literal/1 (quoting identifiers). Also we probably don't want to encourage using this too much as it may blow up the Ecto cache.
• Should we allow more than identifiers and numbers? Then we have an issue where a binary may be interpreted as a literal string or an identifier so we need to add literal/2 or something like that. I'm not sure it is worth it since the use cases are pretty limited.

Companion Ecto SQL PR: elixir-ecto/ecto_sql#651

[josevalim]

I am fine with expanding it slowly as needed, as this looks good to me :)