Add Keyword.delete/3 for deleting explicit values.

[christhekeele]

No description provided.

[josevalim]

Should we also add it to Dict?

[christhekeele]

I don't think so. This PR is kind of part of a general push I want to make for a more useful Keyword module.

My reasoning is a bit verbose and I might post it on the mailing list, but essentially:

• Dicts create strong associations between values and references to them
• Keywords create weak ones
• Dicts shouldn't need by-value access: the key can and should stand in for the value
• Keywords could be more useful with by-value functions: the key is not enough
• Right now Keywords only have all-by-key or first-by-key
• They're essentially second-class citizens as improper Dicts
• By-value functions give them a use-case of their own
• If you need Dict.delete/3, you're using Dict wrong
• Fin

[josevalim]

Agreed.