Arbitrary initial states

**Describe the feature you'd like**

State-loading gate / function - i.e., allowing for arbitrary initializations onto a statevector.

**How would this feature be used? Please describe.**

Circuit().load_to(state)

**Describe alternatives you've considered**

The householder unitary U = I - 2 vv+ achieves this, but requires creating and applying a matrix, both of which are unnecessary and problematic for large n. The QASM implementation seems non-trivial, but I feel there should be a better way.