Handle number->number, any->string, and symbol->keyword

[WhittlesJr]

I'm attempting to use spec-coerce as the backend for cyrus-config: dryewo/cyrus-config#2

During the process, I found a few conversions that I expected to work but didn't, namely:

int? -> double? (remains an int, and thus fails coerce!)
any? -> string? (no string? handler existed yet)
symbol? -> keyword? (can be done by simply calling keyword on it.)

Resolves #25 and then some

[wilkerlucio]

Thanks, looks nice, can you please add tests covering the new behavior?

[WhittlesJr]

Sure! Is this sufficient?

[wilkerlucio]

Sure