Add a compilation error if neither std nor libm features are specified

[LikeLakers2]

Glam would previously cryptically error out during compilation, if neither the std nor libm features were specified. This is due to no_std being turned on, which removed many methods from the f32 and f64 primitives, while glam simultaneously would try to use those methods.

To solve this, I added a compilation error if no math backend is specified.

However, this compile_error!() does not silence the compilation errors generated by the use of non-existent methods. To silence them, I added a new module to glam::f32::math and glam::f64::math - one which defines the methods, but points them all to unimplemented!().

The end result is that, if a backend is specified, glam compiles as normal - but if no backend is specified, glam returns a compilation error:

PS $root\bitshifter.glam-rs> cargo build --no-default-features
   Compiling glam v0.29.2 ($root\bitshifter.glam-rs)
error: You must specify a math backend using either the `std` feature or `libm` feature
   --> src\lib.rs:276:1
    |
276 | compile_error!("You must specify a math backend using either the `std` feature or `libm` feature");
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: could not compile `glam` (lib) due to 1 previous error

PS $root\bitshifter.glam-rs> cargo build --no-default-features --features std
   Compiling glam v0.29.2 ($root\bitshifter.glam-rs)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 8.90s

PS $root\bitshifter.glam-rs> cargo build --no-default-features --features libm
   Compiling glam v0.29.2 ($root\bitshifter.glam-rs)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 8.34s

PS $root\bitshifter.glam-rs> cargo build --no-default-features --features std,libm
   Compiling glam v0.29.2 ($root\bitshifter.glam-rs)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 8.31s

[bitshifter]

Makes sense, thanks!

[LikeLakers2]

No problem! :D