[Lang] [refactor] Separate ti.Vector and ti.Matrix implementations

[strongoier]

Currently ti.Vector is implemented as ti.Matrix with m=1. However, this makes code in other parts unable to distinguish between a vector and an actual matrix with m=1, which causes some ad-hoc checks and parameters when number of dimensions matters. For example, we need to manually specify whether to treat m=1 as an actual dimension when transferring to/from external arrays. To get rid of these, we need to separate ti.Vector and ti.Matrix into two classes.