Turn FluentBundle to be a generic over Borrow<FluentResource>

[zbraniecki]

Fixes #113

[zbraniecki]

The patch is complete. This drastically improves the ergonomics of working with FluentBundle.

One can now do the "simple" thing of pushing Resource to Bundle, or store it in some manager and pass references, or store Arc/Rc and pass them.

This is necessary for https://bugzilla.mozilla.org/show_bug.cgi?id=1560038 to allow me to hook into refcounted WebIDL objects.

I kept the indices, because I don't see a cheap way to avoid them. I understand that they're not pretty, but they're very local and in the current model there is no way to mess with them since FluentResource is immutable after construction and we don't support any other operations except of add_resource at the moment. I hope it's ok.

[zbraniecki]

perf:

     Running /Users/zbraniecki/projects/fluent/fluent-rs/target/release/deps/resolver-84e6f1639e28e79a
Gnuplot not found, disabling plotting
resolve/"simple"        time:   [8.9838 us 9.0265 us 9.0719 us]                             
                        change: [+0.0020% +0.0887% +2.0119%] (p = 0.00 < 0.05)
                        Change within noise threshold.
Found 10 outliers among 100 measurements (10.00%)
  5 (5.00%) high mild
  5 (5.00%) high severe
resolve/"preferences"   time:   [132.89 us 134.58 us 136.97 us]                                  
                        change: [+0.1132% +1.1087% +2.2917%] (p = 0.04 < 0.05)
                        Change within noise threshold.
Found 10 outliers among 100 measurements (10.00%)
  6 (6.00%) high mild
  4 (4.00%) high severe
resolve/"menubar"       time:   [37.254 us 37.435 us 37.629 us]                               
                        change: [-0.0559% +0.8498% +1.7545%] (p = 0.07 > 0.05)
                        No change in performance detected.
Found 6 outliers among 100 measurements (6.00%)
  3 (3.00%) high mild
  3 (3.00%) high severe
resolve/"unescape"      time:   [2.0841 us 2.0939 us 2.1048 us]                                
                        change: [+0.0298% +0.7207% +1.4294%] (p = 0.05 < 0.05)
                        Change within noise threshold.
Found 6 outliers among 100 measurements (6.00%)
  1 (1.00%) low mild
  5 (5.00%) high mild

Gnuplot not found, disabling plotting

[stasm]

Looks great, nice work!