Make extconf.rb executable.

This is mainly for testing, though can be handy if you for some reason
want to go in and rebuild something yourself. I don't anticipate anyone
doing that, but there are crazy people out there.

In this case, I just happen to need it for building the bindings so I
can check for compile errors before building/installing the gem.
Unroll loop in mat4_copy.
Use float_equals in mat4_equals.

It's there for a reason and guarantees the behavior's the same across
all types, so using it makes more sense.
Unroll loop in mat4_multiply.
Remove mat4_copy call in mat4_scale.

It seemed needless to me. Also, write to the output in order as it's
set up in memory.
Add s_sqrt and make all maths macros function-like.

The former was missing and sqrtf was being used regardless of type.

In the latter change's case, it's just to ensure it's being used as a
function and nothing else.
Replace use of sqrtf with s_sqrt.

This'll ensure it's using the right sqrt function for whatever type is
specified on install.
Add mat3_t typedef.
Remove excess modules.

Doing this mostly in an attempt to ensure that doing 'include Snow'
won't drag a bunch of unwanted stuff into whatever it's included in. So
there's not much reason to include something like the MathInspect module
or what have you because it's all pretty specific to the snow-math
types.
Add mat3 implementation.

This is a pretty small amount of functionality, but probably most of
what is useful.
Add mat3 bindings.
Add line-breaks to Mat4#to_s output.

It's easier to read, so that's my criteria for this.
Remove excess line breaks in snow-math.c
Add vec3/4 project and reflect functions.
Add bindings for Vec3/4#project and #reflect.
Version 1.2.0

- Adds Vec3#reflect and Vec3#project
- Adds Vec4#reflect and Vec4#project
- Adds Snow::Mat3 and Snow::Mat3Array
- Adds Mat4#to_mat3 and Mat3#to_mat4 -- these functions aren't yet part
  of Snow::Quat and there are no #to_quat equivalents.
- Mat4/Mat3/Quat#initialize and set all take Mat3, Mat4, and Quat for
  conversion between types.