Support providing numbers with selected precision

[zbraniecki]

Thanks, I can confirm that my previous test no longer panics when using the updated pluralrules.

The thing that struck me as potentially difficult about starting with a float is that when the decimal places haven't been rigidly set by the caller, you'd need to determine how many decimal places are appropriate, and it seems like you'd end up reimplementing the logic in format that does so

assert_eq!(&format!("{}", 3.020000000000000007), "3.02");

The logic looks to be non-trivial:

https://github.com/rust-lang/rust/blob/master/src/libcore/num/flt2dec/mod.rs

Originally posted by @dae in projectfluent/fluent-rs#162 (comment)