Improve performance of record parsing

Using a specialized implementation of sepBy1' improves the performance
on most benchmarks:

Name                                                      Mean
positional/decode/presidents/without conversion            94.07%
positional/decode/presidents/with conversion               87.83%
positional/decode/streaming/presidents/without conversion 106.90%
positional/decode/streaming/presidents/with conversion     89.04%
positional/encode/presidents/with conversion               96.83%
named/decode/presidents/without conversion                 84.36%
named/decode/presidents/with conversion                    92.15%
named/encode/presidents/with conversion                    95.87%
comparison/lazy-csv                                       103.79%