improve the Pattern::add_string ergonomics

[juhp]

I feel it would be nice to hide the CStr's from the API here:

pub fn add_string(&mut self, name: &CStr, val: &CStr)

and have a type for the different kinds of key values, which could then be safely converted internally to an FcPattern.

I am imagining a safe type something like

pub enum PatternKeyValue =
    Family: &str,
    Style: &str,
    :
    Lang: &str,
    Charset: u32,
    :
    Variable: bool;
    :

Any thoughts?

[wezm]

Sounds reasonable. Like a more general version of find, which takes care of the c strings if you're only matching on family and style:

https://github.com/yeslogic/fontconfig-rs/blob/da4c674a2a5b5a04099bd1a9312e090b13865bd1/fontconfig/src/lib.rs#L142