Compact slices when adding many duplicate names

[bep]

Also, simplifying the merge logic.

Handling this case:

info := l.WithLevel(logg.LevelInfo)
for i := 0; i < 99999999; i++ {
    info = info.WithField("name", "value")
}

Which is not the right way to use this API, but probably common enough a case to add a check for.

name                                                       old time/op    new time/op    delta
Logger_context_many_fields_duplicate_names_with_field-10      995µs ± 1%     859µs ± 0%  -13.62%  (p=0.029 n=4+4)
Logger_context_many_fields_duplicate_names_with_fields-10     509µs ± 2%     331µs ± 0%  -35.07%  (p=0.029 n=4+4)

name                                                       old alloc/op   new alloc/op   delta
Logger_context_many_fields_duplicate_names_with_field-10     2.67MB ± 0%    1.53MB ± 0%  -42.84%  (p=0.029 n=4+4)
Logger_context_many_fields_duplicate_names_with_fields-10    1.97MB ± 0%    0.73MB ± 0%  -62.76%  (p=0.029 n=4+4)

name                                                       old allocs/op  new allocs/op  delta
Logger_context_many_fields_duplicate_names_with_field-10      30.0k ± 0%     30.0k ± 0%   -0.04%  (p=0.029 n=4+4)
Logger_context_many_fields_duplicate_names_with_fields-10     10.0k ± 0%     10.0k ± 0%   -0.12%  (p=0.029 n=4+4)