(breaking) vec: Rework vec2.to_vec3; add vec3.to_xz

[idbrii]

• Replace vec2.to_vec3 with vec3.from_table.
• Add vec3.to_xz, vec3.to_xy, vec3.to_table.
• Add vec2.to_table for completeness.

Simplify converting into flat plane math. Often I want to do math on the
ground plane, so it's helpful to easily convert vec3 into vec2 (like for
angle_to).

Remove vec2.to_vec3 so vec3 can depend on vec2 instead and we can do
to_xz and to_xy, which seem far more useful. vec3.from_table(a) is
mostly equivalent to to_vec3, but doesn't accept a value for z.

Add to_table as a pair for from_table and for converting into a
serializable. No vec2.from_table because it's the same as vec2.new.

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What do you think about this one? Maybe all the to/from_table should be removed and just add vec3.from_vec2?

[idbrii]

From the failed test:

Error -> spec/vec2_spec.lua @ 268
vec2: converts a 2-vector to a 3-vector
./modules/vec3.lua:403: 'struct 96' has no member named 'z'

stack traceback:
./modules/vec3.lua:403: in function 'from_table'
spec/vec2_spec.lua:271: in function <spec/vec2_spec.lua:268>

I guess for ffi, I have to make it from_vec2.

[mcclure]

That's rough...
I don't know the best way to handle that. Maybe we could have files with "type only" definitions of vec2 and vec3.