New splitting implemented

[Skillmon]

This pull request ports the changes made to expkv in v1.0 (modulo some corrections) to l3keys's \keyval_parse:NNn. As a result this is 25%-35% faster than the current implementation.

This merge also fixes some minor (up to now) undetected bugs introduced with #678 and some inconsistent behaviour which was already observable with the version prior to that PR.

All these behaviour changes are related to the behaviour of empty or blank key names. This version will throw a blank-key-name error on all of the following inputs (except for the very first key-value list element which is completely blank, this case is still silently ignored as it should be):

\keyval_parse:NNn \use_none:n \use_none:nn
  {
    , {}
    , { ~ }
    , = value
    , {} = value
    , { ~ } = value
  }

[Skillmon]

I'd like to add a few more regression tests here on which I'm currently working (which should prevent things like {}=b from slipping through in the future).