[ufo2fontir] Avoid duplicate loading of kerning groups

[cmyr]

we were calling the (very expensive) kerning_groups_for function twice, whene we can trivially reuse the results of the first call.

this looks to make a measurable difference before/after (in a profile this fn accounted for .6% of the total compile time)

Benchmark 1: target/release/fontc ../fontfiles/googlesans/source/GoogleSans/GoogleSans.designspace
  Time (mean ± σ):      4.327 s ±  0.060 s    [User: 6.830 s, System: 3.038 s]
  Range (min … max):    4.252 s …  4.443 s    10 runs


Benchmark 1: target/release/fontc ../fontfiles/googlesans/source/GoogleSans/GoogleSans.designspace
  Time (mean ± σ):      4.282 s ±  0.044 s    [User: 6.835 s, System: 3.034 s]
  Range (min … max):    4.218 s …  4.349 s    10 runs

(JMM)