Bundled FluentMessage and FluentArgs type

[clarfonthey]

Essentially, I want a pair of these that has been pre-validated, but not actually formatted into a string. This should just involve verifying the type and existence of arguments without actually formatting them inside the string.

Right now, the only way to actually "verify" this is to format the string and toss out the result, which is a pretty bad API IMHO.

[gregtatum]

Seems useful to me.