the length check was running sequence checks on every number which would fail, small speedup by avoiding this.
should eventually get this working faster by reading once into an allocated array.
- rna array parsing was using PySequence_Size() in a loop, this can be slow to run so only call once.
- assigning a single value to a multi-dimensional array was missing type check.
- improve exception messages for rna array type errors.
- simplify vector slice assignment by using mathutils_array_parse(...)
