Make more Transform APIs interoperate with Dir3

[mweatherley]

What problem does this solve or what need does it fill?

Presently, Transform::look_to and Transform::align (along with other APIs derived from them) take Vec3 inputs even when their parameters are, morally speaking, directions. Conversions between Vec3 and Dir3 are annoying for end-users, and some of our other APIs provide Dir3, while doing mathematics 'by hand' often results in a Vec3, so simultaneously capturing these would be nice.

What solution would you like?

After some discussion on Discord, it appears that using impl TryInto<Dir3> would be natural for these cases, since they

1. treat their input essentially as a direction even though it's a vector
2. already provide replacement input in the case that the conversion Vec3 -> Dir3 would fail

Thus, in particular, changing these inputs from Vec3 to impl TryInto<Dir3> is not even a breaking change, but it would allow Dir3 input to be given just as easily as Vec3.

What alternative(s) have you considered?

Users can manually convert between Vec3 and Dir3 to get around this.

Additional context

On the level of API design, just using Dir3 was considered for Transform::align because it is more "natural", but providing a Dir3 is often not ergonomic (especially since the Vec3 conversion is fallible) and because Transform::look_to used Vec3 already.