0.16 rc1 - No way to turn on bevy_math/curve through the bevy crate

## Bevy version

0.16.0-rc.1

## What you did

Depend on Bevy without default features, but with `std` enabled (which enables part of `bevy_math`)

## What went wrong

There is no way to enable the `curve` feature on `bevy_math` through the `bevy` crate. `bevy_internal` optionally depends on `bevy_math` without default features and there is no feature flag that would turn on the `curve` feature.

## Additional information

The obvious workaround is to depend on `bevy_math` directly.

Start by inspecting the Bevy crate's feature definitions and the optional bevy_internal dependency configuration. Trace how the std feature forwards features to bevy_math, then verify that enabling a corresponding Bevy feature activates bevy_math's curve feature without default features.

Written by the indexing model from the issue text.