v6.9.0: Reorganize layers, bug fix to Layer Normalization

✨ Major features and improvements

• Add new namespace modules thinc.v2v, thinc.i2v, thinc.t2t, thinc.t2v that group layer implementations by input and output type v indicates vector, i indicates integer ID, t indicates tensor. The input type refers to the logical unit, i.e. what constitutes a sample.

🔴 Bug fixes

• Fix bug in layer normalization. The bug fix means that models trained with Thinc 6.8 are incompatible with Thinc 6.9. For convenience, a backwards compatibility flag has been added, which can be set with thinc.neural._classes.layernorm.set_compat_six_eight. This flag is off by default.