It's not a submodule. It's a module that sits next to the stdlib. There's a working implementation linked in the proposal that you can experiment with.
This was addressed in the SIMD proposal thread: SE-0229 — SIMD Vectors - #112 by scanon. This would be the first library that's "distributed next to the standard library" (the third point there), but the first two points still apply:
- We need the SIMD types in the stdlib because we want to use them to implement features in the stdlib. That's not a concern for the Math module because we have the escape valve of the implementation hooks
T.Math.exp( )that we can use in the standard library. - The functions defined by the Math module don't change how (or if) other modules are imported, whereas a module containing the SIMD types would have.